Thank you for the clear instructions which i followed to the dot....

Suggestion 1 (switching to protostar) -> nope, no success
Suggestion 2 (updating all plugins) -> nope, no success (but was a good idea anyway)
Suggesion 3 (checking console) -> heureka ... "too much recursion"

More detail: components/com_comprofiler/plugin/user/plug_cbconditional/js/cbcondition.min.js?v=73c3597ef457e5fb:1
(basically reapeted a number of times)

I guess I need to check the cb_condition plugin for some misuse... Will try and report back.

Avatar did not have any changes for "conditional" - error persisted.

Okay ... unpublishing the plugin "conditional" resolved the issue. However - the following warning still appears in the console:
JQMIGRATE: Migrate is installed, version 1.4.1 jquery-migrate.min.js:2:542
JQMIGRATE: Migrate is installed with logging active, version 1.4.1
JQMIGRATE: jQuery.attrFn is deprecated
Might this be related to the problem?

Next I will need to check, whether conditional is still needed somewhere else in my setup (or can be deleted)

And perhaps you might want to look at why "conditional" acts up in this way... I simply have no clue.

Thanks