I decided to migrate the existing site from Joomla 4 to Joomla 5 and caused the following error in the Administrator Panel:
0 Class "JPlugin" not found
I enabled debug and see this one:

This result I get when I proceed to System > Global Configuration or System > Extensions.
The content plugin causes this issue on the backend?
There are the following extensions installed on the site:

It seems the plugins (content and other) need to be adapted to work on Joomla 5 even with Backwards Compatibility plugin.
Thanks.
UPD: I thought Backwards Compatibility plugin was enabled after the update, but it did not. By enabling it the problem could be resolved. However, it is better to adapt PD's plugins someday.