Solved Could not instantiate mail function.

Since the last 4 days whenever somebody posts a message there is no longer mailing to the subscribed users.
An error messages appears : Could not instantiate mail function.

I'm using Kunena on Joomla 3.10. The test mail in Joomla works fine.
Mailing from other components (contact page) works on the joomla website.

How can I find the root cause of the problem?

Configuration:

Database collation check: ✔ The collation of your table fields are correct
        

Joomla! SEF: Enabled | Joomla! SEF rewrite: Disabled | FTP layer: Disabled |
        

This message contains confidential information

htaccess: Exists | PHP environment: Max execution time: 300 seconds | Max execution memory: 536870912 | Max file upload: 256M

Kunena menu details:

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ID	Name	Menutype	Link	Path	In trash
614	LedenForum	mainmenu	view=home&defaultmenu=231	ledenforum	No
124	Forum	mainmenu	Itemid=216	kunena-2016-02-09	No
268	Leden	mainmenu	view=user&layout=list	data-doc/leden	Yes
216	Forum	kunenamenu	view=home&defaultmenu=218	forum	No
217	Index	kunenamenu	view=category&layout=list	forum/index	No
218	Recent Topics	kunenamenu	view=topics&mode=replies	forum/recent	No
219	New Topic	kunenamenu	view=topic&layout=create	forum/newtopic	No
220	No Replies	kunenamenu	view=topics&mode=noreplies	forum/noreplies	No
221	My Topics	kunenamenu	view=topics&layout=user&mode=default	forum/mylatest	No
222	Profile	kunenamenu	view=user	forum/profile	No
223	Help	kunenamenu	view=misc	forum/help	No
224	Search	kunenamenu	view=search	forum/search	No

Joomla default template details : J51_Classic | author: Joomla51 | version: 1.2 | creationdate: July 2014

Kunena default template details : Crypsis | author: Kunena Team | version: 5.2.6 | creationdate: 2021-07-25

Kunena template params:

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DefaultCategoryicon
profileIconset	default
DefaultIconset	default
editorIconset	default
bootstrap	0
icomoon	1
tooltips	1
borderless	1
SubjectLengthMessage	80
fullactions	1
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displayDropdownMenu	1
displayDropdownContent	1
displayModule	1
displayBreadcrumb	1
displayAnnouncement	1
displayFooter	1
avatarPosition	left
avatarType	img-circle
labels	0
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topicicontype	B2
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socialsharetag
socialtheme	classic
optional_username	0
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IconColorNew	#48a348
editor	0
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Smiley	1
Confidential	1
Hidetext	1
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Code	1
Ebay	1
Twitter	1
Instagram	1
Soundcloud	1
FontSize	1
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ckeditorParams
wysibbParams
nameskinckeditor
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avatarSizeX	40
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avatarSizeYList	36
avatarSizeXPost	144
avatarSizeYPost	144
avatarSizeXProfile	200
avatarSizeYProfile	200
templatebyText
templatebyName
templatebyLink

Kunena version detailed: Kunena 5.2.6 | 2021-07-25 [ Melia ]
        | Kunena detailed configuration:

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Kunena config settings:
board_offline	0
enablerss	0
threads_per_page	40
messages_per_page	6
messages_per_page_search	15
showhistory	1
historylimit	6
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template	crypsis
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showchildcaticon	1
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enableforumjump	1
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username	1
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useredittimegrace	600
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allowfavorites	1
maxsubject	63
maxsig	300
regonly	0
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floodprotection	0
mailmod	0
mailadmin	0
captcha	0
mailfull	1
allowavatarupload	1
allowavatargallery	0
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avatarsize	2048
imageheight	800
imagewidth	800
imagesize	2500
filetypes	txt,rtf,pdf,zip,,tgz,
filesize	20000
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trimlongurlsback	20
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imagemimetypes	image/jpeg,image/jpg,image/gif,image/png
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avatarcrop	0
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attach_end	14
attachment_utf8	1
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quickreply	1
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activemenuitem
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recaptcha_theme	white
keywords	0
userkeywords	0
userlist_name	1
usernamechange	0
version_check	1
userlist_usertype	1
sefutf8	0

| Kunena integration settings:

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Kunena - Finder Disabled
 Kunena - AltaUserPoints Disabled
 Kunena - Community Builder Disabled
 Kunena - Easyblog Disabled
 Kunena - Easyprofile Disabled
 Kunena - Easysocial Disabled
 Kunena - Gravatar Disabled
 Kunena - JomSocial Disabled
 Kunena - Joomla Enabled:  access=1  login=1  
 Kunena - Kunena Enabled:  avatar=1  profile=1  
 Kunena - UddeIM Disabled

| Joomla! detailed language files installed:

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Joomla! languages installed:
en-GB	English (United Kingdom)
nl-NL	Nederlands (nl-NL)

Third-party components: None

Third-party SEF components: None

Plugins: None

Modules: None

Have you changed anything on your website in the last four days?

I noticed the problem yesterday. First things I did was check if there were new versions for the Joomla software and Kunena software.
I did an upgrade from Joomla 3.9.x to 3.10.x
I did an upgrade from Kunena 5.1 (not sure) to 5.2.x

Besides those updates no changes on the configuration. I do not know if my hosting company did any changes but I do not think that the cause of the problem is on there side because the test email from the joomla website does work (and from the contact page).

After the problem still the same issue.  

I did try changing the email config from phpmail to smtp (sendmail not allowed by hosting company).
But then I ran into other problems : The following From address failed: xxxxxx@: MAIL FROM command failed,,, 
I did masked the email from address
Besides that it did work fine with php mail ...

I did enable kunena debug mode but I did not notice any other error messages

I did search this forum and the internet but didn't find an example of the same issue in recent history. 

You have a valid email address in the  field "Visible E-mail Recipient" in the configuration -> (tab Users)?
Check also your spam folder, maybe the mails are handled as spam.

Hi Rich,

Yes the field "Visible E-mail Recipient" contains the same e-mail as configured in the mail settings for Joomla.
And there are no e-mails in the inbox (and not in spam). 

Is it possible to enable more logging to find out what is the problem? I tried debug logging in Kunena but did not find new/more information.

Best regards, 

Dear all,
I do not know what happened but I did not change anything but when testing now. I do not get the error and messages are delivered.
Very strange ...

Good for me but no root cause :-s

Yes, there are such mysterious errors where you can't figure out what triggered them.