I encounter the same issue and same error code 909997.
My version is 6.0.6 (323) on a W10 1809 build 17763.379 and sync towards a Nextcloud WebDav.
If I remove and re-configure the sync, only the initial sync is running, nothing after.
I have no issues syncing from Linux and Android towards this WebDav.
As it only happens on the W10 machine, I guess there is something with Enpass on W10, as all my other devices work like a charm.
Did you managed to solve the issue on your end?