Question PHP Fatal error: Cannot access empty property in /libraries/joomla/registry/registry.php on line 194

I don't know where is the cause of this issue

Do you looked for backup component (akeeba..) ? I still advise you to upgrade Kunena to 1.7.1 and module kunena latest to 1.7.1 when you have upgraded Kunena.

I've been searching for information about this error and here's some of what I found: >>> here <<<

When you look at everything together, here's my assessment of the situation:

(1) The error is generated by PHP; in this case the site is using PHP 5.3.8. It's possible that PHP 5.3.8 is less-tolerant of deprecated PHP code that may exist in J! 1.5.25 and that something needs to be done to fix Joomla. That's a possibility we can't dismiss. In this case, it's unlikely that upgrading to K 1.7.1 will fix the problem. It's also possible that a simple change to the configuration of PHP 5.3.8 may fix the problem, too.

(2) It would be better to fix the cause of the HTTP error before attempting to upgrade.

(3) The problem may exist because of K 1.6.4 but we can't tell for certain. K 1.6.4 was released on 7-May-2011; PHP 5.3.8 was released on 23-Aug-2011. Unless someone else has a test site running the same software as Helene, we have no way to reproduce the situation. In this case, it's possible that upgrading to K 1.7.1 may fix the problem.

(4) The problem may be caused by ExtendedReg. I don't know anything about it. It may be worthwhile to see what happens without ExtendedReg.

(5) It more likely that the problem occurred not because you did not upgrade Kunena but because of something else.

(6) Xillibit has given you good advice: you should backup your site before you try to upgrade K 1.7.1 (and Kunena Latest 1.7.1 later on). It is very unlikely that you will lose any posts on your forum as a result of upgrading.

Dear sozzled. THANK YOU very much.

Though its not of course solved, but thank you so much for this you have looked up. This gives me something to go on with, - so thank you again.

I think that this bug is usually caused something else than Kunena, perhaps module or a plugin. It is very hard to locate, but disabling modules and installed plugins one by one sometimes solves the issue.

Try to remember what you did just before this message started to appear, and you have probably solved the issue.

Thank you matias for your comment, - this may be something that I could have tried.
However, I went back to Joomla with sozzled's observations and after some days they have fixed it, and returned this to me, which he suggested I might post here:

Some technical details (you may want to post it on Kunena forum):
Kunena is calling JRegistry->setValue() (registry.php:194) with $regpath set to com_kunena..limitstart. It should be com_kunena.limitstart. My workaround removes this extra dot.

However, what is strange, is really that I have never had this before, - it came just suddenly, - so having worked fine for a while, it suddenly didnt work, and this should rather indicate as you say, that I have done something just prior
And I do know that that was to update Extended Registration.

Right now the issue is solved, but it will probably return to it with the next Joomla update, - so I still need to do something. My next move will be to update Kunena forum.
And I think I will remove Extended Registration in the same move.

I hope maybe this information is useful for you.

Thanks for for your update. Would it be possible for you to post the URL from the Joomla forum where you obtained that information, please? I tried to look for your discussion on the Joomla forum, using keywords from this topic, but I was unable to find it.

We've run a check through all K 1.7.1 code and we cannot find a match for the condition you reported. Therefore, the possibility exists that the problem lies with the version of Kunena and add-ons that you have installed and it's also probable (in spite of what I wrote earlier) that upgrading Kunena to version 1.7.1 (as well as any Kunena add-ons you are using) will fix this issue. We also do not remember seeing reports of this error in in K 1.6.4.

I think the problem for us was that you posted this topic in the K 1.6 and K 1.7 Installation and Upgrade category - which is where people ask questions or report problems they've encountered during installation or upgrades of Kunena. I now understand the purpose of your initial query which was to ask "if I upgrade Kunena will I lose anything?" The answer to that question is [in 999 cases out of 1000] no, you will not lose anything.

The problem exists, in this case, probably because of K 1.6.4 which, as we all know, is no longer supported. What is the most likely cause of the problem now appearing is that something else happened after you installed Kunena to make things happen like they did (the PHP upgrade may be one possible cause, we don't know). So, in this case, I think that xillibit was right (and my conclusion (3) written earlier was also right) - that you should upgrade to K 1.7.1