recovery mode – TWRP fails to mount data partition with error, says Invalid argument

I have a Fairphone 4 with /e/OS that I accidentally left a long download running overnight until the storage filled up. The phone also ran out of battery during this, and now it won’t boot. The behaviour is always the same: It makes it all the way through the boot screen and to the initial unlock prompt, but when I enter my pin, the launcher fails to load and it shuts down by itself. It reboots into recovery, but the default /e/ recovery is really limited and only has the option to wipe the device and no file manager.

I really don’t want to wipe as I have a lot of data on the phone so I flashed TWRP onto it to try to access the filesystem and delete the offending file. However, TWRP cannot seem to mount the /data partition, neither from the mount screen nor the terminal. Both cases it gives the following output

mount: ‘/dev/block/sda11′->’data’: Invalid argument

The phone is encrypted. Is there any hope of getting the phone to boot properly again at this point? Or is my only option to wipe? Also, because it can’t mount the data partition, it’s not giving me an option to back up the storage, is there still a way to do that and decrypt the output? The data is really what I care about.