Group of unsolvable API and core errors

Question

I really would appreciate any advice to dig deeper into the debugging of some errors. The reason may lay in our installation which is grown since late 2015 (I think it was version 4.2) and widely customized. But the errors appear in combination with some newly installed extensions.

1. civi expenses: when I try to add an expense, I get an ajax error Error: Class name must be a valid object or a string in Civi\Api4\Service\Spec\SpecGatherer->getDAOFields() (Zeile 168 sites/all/modules/civicrm/Civi/Api4/Service/Spec/SpecGatherer.php).
2. Same when adding a new dataprocessor Error: Class name must be a valid object or a string in _civicrm_api3_basic_create_fallback() (Zeile 1347 von sites/all/modules/civicrm/api/v3/utils.php).

In both cases it seems an api call is not registered correctly - v4 in the former and v3 in the latter).

I flushed all caches a thousand times.

The extensions and their db updates are up to date as the core's. The status page does not list any errors. I de- and reinstalled extensions and deinstalled old and unneeded extensions.

I tried to produce some stacktraces with drupal's devel module but w/o success. And I did ask the extensions' author about ideas but no luck as well.

php is 7.2, our civi ist 5.37.2, drupal 7 is latest.

We are desperately willing to buy consulting hours.

Answer 1

Turns out ot was a php function call outside class methods in an old extension which used to work before but now caused this weird behaviour.