I have observed this problem on one of my devices.
Basically, Syncthing seems to often hang when trying to add or remove a folder. The problem is that once this has happened, both the GUI and the process seem to stop working for about 3-5 minutes. The log file stops recording anything, and the CPU usage drops to almost nothing. Then, after a few minutes, everything suddenly comes back to life.
This is how it looks like in the GUI.
I have also found these kind of entries in the log file, which correspond to when the problem occurred.
[TLFGV] 07:55:24 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:36 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "7LDIBZ4"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:36 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "7LDIBZ4"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:54 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:04 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:07 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:13 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:24 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
This particular hang occurred after trying to add a folder shared from another device. What is interesting is that despite the errors, after Syncthing had came back to life, the folder did appear in the GUI, correctly added.
I am assuming that this may have something to do with the connection timeout or something related to it, but this is just my guess for now.