Hi Petro,
Zeiss Jena microscopes, especially the Microval series, often suffer from hardened grease.
It helps to gently warm the instrument after removing the optical components. An electric room heater is useful here. Be careful with heat guns and hair dryers, the are not always up to continuous use.
You might place the question in the german
www.mikroskopie-forum.de . There are people there who have worked on these microscopes.
Bob