I think I have found what causes this error. It has to do with the LAM (LibAddonMenu).
Apparently, something called Libstub has become obsolete, which means add on developers need to change the way their add on calls on libraries or the menu settings don't work anymore. (I am not a developer so I may have scrambled the information a bit or not used the right wording, I do apologise for that!). Until all add ons are updated by their developers to reflect this change, the workaround seems to be to install the LibStub library yourself. It will give an in game warning about it being deprecated, but my add ons show up in the settings menu again now.
More info here: https://www.esoui.com/downloads/info7-LibAddonMenu.html#comments
And here: https://www.esoui.com/forums/showthread.php?t=8765&highlight=LibStub