Thank you all for the answers, hm, bad news here, which reads: "error 10" ;-(
1) I have done this "clean and rebuild" several times without success
2) As i know myself as DAU, I backup my files every some hours in a complete directory to rename and switch back. Although I want much to avoid this at the moment, because I would lose - some hours of work.
3) Very interesting is the thing "look for not wanted items in your list". I see a ghost called datamodule1 on clicking "Forms", there it is listed. I have no idea, where it comes from. There seems to be no unit or ressource connected to it. this "datamodule1" is not in the list of project contents neither.
4) I do not work with tvPlanit at the moment. In this project, I will generate the calendar frame later. The tricky thing is, that I have a looong TPageControl with one or two frames for every Tabsheet. Those frames are all nested. I would like to clean up those uses clauses: However, I do not know how. Interface and implementation: there are uses clauses to lots of frames. And I do not think, there are too many not needed.
5) Once upon a time, in a country far away, I had such a problem with Delphi. An error-message about an element I could not find anywhere. I used a text-editor to find it in a file and just forced it by ASCII. Unfortunately I cannot remember which was the name in Delphi, nor I have an idea, if there is such a file in Lazarus and how it may be called.
Perhaps I can start it by the question:
This "datamodule1", where may it come from?
and PS:
"datamodule1" is not found by a text search in any file of my project.