How to Solve Block-Editor Problems?

I haven’t been successful using the ZATZLabs-ticketing system, so, I try it here:

It looks like I have problems with the widget editor and the block editor when using the Multiple Themes plugin.

I have the main pages of the site developed with the Total Theme. However, as now there should be an integration of the LMSLifter learning platform which does not like Total Theme, I have switched to the OceanWP theme (recommended by LifterLMS) and I use Multiple Themes for keeping the existing Total Theme-pages operational (until they are changed to the new theme).

However, I see that the widget editor can not be switched to the block editor mode. That mode is the normal, required mode for the LifterLMS theme. Before I tested Astra theme with the same effect (while the widget editor works well in a separate WP installation without Multiple Themes plugin). At the same time – but I am not so sure about this – I suspect Multiple Themes to be the reason for missing some features in the regular block editor for single pages. (for example I am missing theme buttons)

Are there any such experiences with other users and what can be done about this?

The page I need help with: [log in to see the link]