Dozens of “permission denied” errors and all local folders STOPPED after upgrading the QNAP .qpkg from SyncThing_1.27.1_x86_64 to SyncThing_1.27.9_x86_64. QNAP is TS-253D running QTS TS-X53D_20240605-4.5.4.2790.
Adding the (new?) syncthing user to the admin group and restarting syncthing did not fix the problem. Shared folder permissions shows syncthing having read/write access to all synced folders. All OK after downgrading to SyncThing_1.27.1_x86_64, which runs as admin.