Unable to save resume file - no space left on device

[frankhovin]

This message suddenly appeared in my web gui:

Error: Unable to save resume file - no space left on device

However, there is lots of space on my device:

$ df:

/dev/sdc1 961300808 448605940 463840496 50% /mnt/torrents

Here's my folder structure (transmission downloads to a USB disk - the system is Ubuntu Server 16.04):

frank@odin:~$ ls -la /mnt/torrents/transmission/
total 28
drwxrwxr-x 5 debian-transmission debian-transmission 4096 Dec 8 10:06 .
drwxr-xr-x 5 root root 4096 Dec 13 19:00 ..
drwxrwxr-x 62 debian-transmission debian-transmission 12288 Dec 25 02:56 completed
drwxrwxr-x 2 debian-transmission debian-transmission 4096 Dec 8 10:06 incomplete
drwxrwxr-x 2 debian-transmission debian-transmission 4096 Dec 8 10:06 torrents

Anyone got any ideas why transmission suddenly believes that there's no space left on a device that's 50% full? I've even deleted several GBs of data and restarted the transmission-deamon service. Didn't help a bit. The error occurs for several existing torrent downloads, as well as new ones I start. Sometimes, when I right-click and select "resume now", the torrent will start again for a few secons, and then stop again with the same error.