Thank you for all your answers!
I share the assumption that the endless loop during loading is caused by dependent forms! The project uses a lot of TFrames, sometimes
frames within frames placed on a form.
The project is 17 years old and was converted from Delphi 7. Today we would solve many things differently, but the old project is too complex and too old for fundamental changes. I am not allowed to send the source code.
Regardless of the old project, the endless loading loop of the IDE (displaying thousands of forms again and again) is a bug. Fortunately, it occurs only occasionally.
I only use a pure Lazarus installation (installer from your website), without any external components! With every new Lazarus version I delete all old settings, so that I can exclude any external influence.
But as long as I am the only known user with this problem, it can be ignored. I can handle it.
Have a nice weekend!