finally both hosts have agreed they are back in sync.
I did have to force a shared folder, by renaming it on one hosts in the filesystem.
The lesson learned : should’ve paused all folders from the get-go, and resume them 1-by-1.
Much easier to see what’s (not) happening.
Thank you to @imsodin & @calmh for advising in this thread.
Thank you to all developers, for this great tool call Syncthing.