Is device “G91S” your Android phone?
Unless there are two difference devices labeled as temp-OptiPlex-7010, one issue I see is that according to your screenshots of Syncthing-GTK, all of your configured sync folders are being shared with device temp-OptiPlex-7010. However, that device is also the one the screenshots were taken from. Or in other words, isn’t /home/mel/Sync already on device temp-OptiPlex-7010?
/home/mel/Sync/German/Berlin Reise -- 2023 is shared twice, which is fine (the two folder IDs are unique), but you’re receiving changes from both device G91S and Schmetterling. Device G91S knows the folder ID as mdkr4-sjfbx while device Schmetterling knows it as pldr7-8sxcx, so as far as those two devices are concerned, they’re writing to different folders and potentially causing conflicts. Instead, if the goal is to prevent changes from being synced from temp-OptiPlex-7010 back to Schmetterling, share mdkr4-sjfbx with Schmetterling and set the folder type to “Send Only” on Schmetterling.
As for the error “.stwritetest: syncing: finishing: pull: no such file”, the official Syncthing Android app creates the temporary file to test that it has enough permission to write to a chosen storage location. See the following earlier post for additional details: “syncing: pull: no such file”, this time on a Syncthing file
(If the “Introducer” setting on Schmetterling was enabled on purpose, I recommend disabling it until you’re much more comfortable working with Syncthing. Automatically pushing devices and folders to other devices requires extra care to manage properly.)