Discovered and can be reproduced with DavDroid 0.8.0, which is the first version using REPORT.
On some calendar URLs, although the PROPFIND succeeds
"PROPFIND /calendars/greve@georggreve.net/c81425b202e6-3182008d3351-2712f63c/ HTTP/1.1" 207 1018 "-" "DAVdroid/0.8.0"
a REPORT on the same URL then fails with 404
"REPORT /calendars/greve@georggreve.net/c81425b202e6-3182008d3351-2712f63c/ HTTP/1.1" 404 739 "-" "DAVdroid/0.8.0"
See https://github.com/bitfireAT/davdroid/issues/542 for more information.