TOPIC:

For me it looks like an inlcude path is broken.
/is/htdocs/######/libraries/src/HTML/HTMLHelper.php ist not a directory where JEM files could be found.
They are all in a "/../com_jem/../" directory.

Which part of JEM produces the error?
Is it the calendar view, eventlist, or a module?

Can you test it going back to PHP 7.4?

The problem also occurs on PHP 7.4. When calling the JEM - module wide and the menu item category view.

Sorry, I can't help anymore. In my eyes, you have to go back to a backup which worked.
For me it is strange, that you use the wide module on a small topic! And jem wide module look much other.
I don't know too which template this is and if there are any interferences.
In my cases this never happens using protostar and it never looks like that. You must havve done something irregular which is causing this.
I renounce.

Hello,
I just updated JEM to 2.3.5. Now everything works again.
Many thanks for the support.
Wilfried

Very interesting,
there should be not difference between 2.3.4 and 2.3.5.
But if it works for you, everything is fine