The error says that on C: the total available space is 0.8%. Either you create some free space there or you configure Syncthing to require just e.g. 0.5% free disk space (you can also configure an absolute amount, e.g. 5GB).
What do you mean by “I increased the disk space for Syncthing” - what did you do, concretely?
What you quoted just exactly exemplifies what I explained: 0.8% free on disk is less than both 1% or 2% (required by Syncthing).
Free disk is necessary for the creation of temporary files for many things in your OS. My Windows systems often start to exhibit weird behaviour with anything less than 5 GB free disk space. The other commenters are correct that your only options here are to reduce your Syncthing allowed space to ~0.5% or to increase your storage capacity. I would strongly recommend the latter.
Thanks, that is helpful. I deleted a lot of files on my harddisk, now syncthing has synchronized both systems.
Now one of the two systems reports “up to date”, the other one reports “out of sync” and lists 68 files. What can I do to get the status “up to date” on this system, too? I don’t need the mentioned 68 files any more, but it would be a lot of work to delete them by hand.