The Kunena team has announce the arrival of Kunena 6.3.0 [K 6.3.0] in stable which is now available for download as a native Joomla extension for J! 4.4.x/5.0.x/5.1.x. This version addresses most of the issues that were discovered in K 6.2 and issues discovered during the last development stages of K 6.3
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I do not know if the error has got anything to do with Kunena. The error occurred twice. First time when storing a Category under a new Parent. Second when removing a topic on the front page.
Should have added that I havn't noticed this error before.
The bug is not related to Kunena in any way, so no configuration report is needed.
You have PHP 5.4 installed and something in your site is triggering this error (likely trying to set PHP directive that's not there anymore).
The error is already as detailed as it gets. The error is not happening on any file but most likely inside eval(). That could mean that the site has been hacked or you just have badly written extension (plugin?) installed into there. It could be almost anything.