I suspect that is the problem then - the file name is too long after encryption
I realise that I misinterpreted the previous log - there are 405 errors for MKCOL but rclone was dealing with them. The 405 errors that get returned to the user are those for PUT requests of these encrypted file names
/remote.php/webdav/RCCrypt/mmivc4rrflegv299dmj9ocreb8/sanpd8aaer0972qqeventh2a3c/s6kuh5vup5bsigdtrfp0hkd9abhch3qs7fvrj2i1siesftofm23751mudg53fqc89s5umqev4cm6v3d4mc2k2sf97grlc6e7g1tcergel8d6uk6tdhc76gmbgoi5h527q28nmj0gbpcel3rpv0gptl1oro
/remote.php/webdav/RCCrypt/mmivc4rrflegv299dmj9ocreb8/sanpd8aaer0972qqeventh2a3c/ba0d1876tpe13buuv1i5r3hm7alght9ahj9hlk58gft0otgdl2hn0ouqi73vaqaetesjucf3b9nq03tbcrq328ko1iuqv97eot5nli73nqvie1ikqbdu2nsrcslf4pk93vomn7nodb8clbh6jr2irrbpbg
/remote.php/webdav/RCCrypt/mmivc4rrflegv299dmj9ocreb8/39prer1b5ho916jhnbkbqjr8vg/gqi93447tlbgrl5tbu8rjalq30d6lurc7nifteqn5pg5ma758ljn6461ltpupsh97jcnauj7gfanl5usi866gfq08nf8thudijrgubloh0dqqdrckf44hsesq3n49vijj6agahn87u6empe4m8nqbeceg0
These file names aren't extremely long - 235 characters in total with the longest path being 154 characters
Can you run this
rclone info --check-control=false --check-length=true --check-normalization=false --check-streaming=false StackCrypt:rclone-test
And rclone will print how long the longest file name it can make without an error is - this is what I get on my local file system.
// local
maxFileLength = 255