Seems to be fixed in 9.0.3. Although I am unable to pinpoint the ticket that solves this issue in Issues resolved in 9.0.3 | Confluence Data Center 9.1 | Atlassian Documentation.
EDIT: Sorry, seems to have made some mistake. Checked 9.0.3 today again, and the error is still there. The page can be saved with some message in the UI and there is the error message (with stack trace) in the logs.
EDIT2: Still an issue in 9.1.0? See Confluence 9.1 release is available now - #23 by rr1