fsarchiver backup fails due to write error

[deafmute1]

getting an error like the following, on and off:

[2024-04-06 22:00:06+1100] [backup] Starting FSArchiver backup...
archwriter.c#200,archwriter_write_buffer(): write(size=130026) returned 437
archwriter.c#209,archwriter_write_buffer(): Can't write to the archive file. Space on device is 3.53 TB.
If the archive is being written to a FAT filesystem, you may have reached
the maximum filesize that it can handle (in general 2 GB)
oper_save.c#223,createar_obj_regfile_unique(): operation has been interrupted

No idea what this error could be caused by.

Its writing to a mergerfs filesystem which overlays individual ext4 partitions.

Plenty of space on the fs as seen above.