I think I found the thread with same issue that you cited: this
Just for update:
Nothing worked: permissions, uncompress folder, remove folder and readd it, activate long paths in Win10 group policy (now I doubt this has to do with long paths, since I found one short). Instead, I found that some files inside the directories with problems are not being synced. Does not suffice to delete the directory but the whole branch must be deleted.
Moreover, global status is different in the two devices, which seems to indicate that the problem relays in the global index synchronization.
Send Folder, gobal and local status:
Receive Folder, global and local status:
When changed the receive only folder to receive and send, the status changed to synced, and when turned again to receive only, stayed synced without the files that where missed.
When a branch with missing files is deleted, it gets the directory structure and files that where not missed again, but no the missed ones. Definitely this seems to me that has to do with the index.
Syncthing 0.14.2