Good morning. I had this same error with both my PC and Android on DSM 6.2 and DSM 7.0, some time around June/July 2021 syncing stopped. After the upgrade to DSM 7.0 I thought that would fix the issue. I looked to convert the permissions to ACL as noted here but tried something else first. What solved my issues was leaving the permissions as they were but I went into the file station, to the webdav folder, I right clicked on the folder where I store Enpass, went to properties, and checked, "apply to this folder, sub-folders and files" under owner this re-pushed the folder permissions down again and after I did that everything started working again.
It appears an update may have broke the permissions on the webdav folders, that's why converting to ACL or re-pushing permissions seem to work.
Just passing along what worked for me.