Have you activated Last Poster Avatar?

Second error is from template showthread_userbrowsing_user. Try and revert back to original using the 'options' tab.

Have you activated Last Poster Avatar?

How to? 
I've found a plugin named "Last Poster Avatar".
I download it and installed it from:
https://community.mybb.com/mods.php?acti...oad&pid=74

Now I have a bunch of error messages... Even on the plugin page of the Admin panel...
I uninstalled this plugin, reinstall the plugins from your theme (I noticed there are a few .php file in inc/plugins).
It ended like this:

(the plugin page in Admin panel has errors if some plugins are not activated)





Then, after activating all the plugins, I got this... 
(The picture is too big for Imagepost, screenshot uploaded on WeTransfer)

https://we.tl/t-eHMDjMuRss

OK, I found the culprit: 

The Thread prefixes plugin.
When it is activated, there is no error showing on the plugin page of the Admin Cpanel. But tones of errors at the top of each page of the forum.
When it is disabled, no more error on the forum, perfect! But there are errors messages showing on the plugin page of the Admin Cpanel...

Undefined array key "tpref_enable" - Line: 16 - File: inc/plugins/tpref.php PHP 8.1.27 (Linux)U
Undefined array key "tpref_enable" - Line: 20 - File: inc/plugins/tpref.php PHP 8.1.27 (Linux)