Hi Daniel,
we had an issue very similar to yours and got the same error message. However our message was triggered by a formatting issue of the Documentation. The formatting is most likely introduced by Version 21.0.9HF1 as since this Version we are encountering it too.
So the issue lies within how the object is stored. You can see it by exporting the object via the Context menu and checking out the .xml file it generated. Our issue was within the <DOCU_Docu> tag. While not being fully relevant to your issue, I will explain a bit further. Maybe it helps you to find out the issue in your .xml defintion.
We had a job that had a Documentation Content node. Since this version, some tags seem to be written differently. Here you can see that <Content> is getting written into one line. 
However a javascript of the AWI does not like that and does not even trigger a request to the AE. Hence why there is zero logs.
If you go to the .xml and edit it like this and re-import it, the object can be saved again:
I tried to manipulate the specific Calender you mentioned on our Dev system but I could not find any constellation that triggers that error for our side as well. But maybe you can find anomalies in the .xml Export.
Maybe as a reference you can revert your Calendar to the initial version, export it and compare both .xml files.
Good look to anyone fighting this issue.
Cheers, Nico