I get a similar error i.e.:
RPC ERROR: Filesystem I/O Error /media/embassy/tmp/(random characters)/EmbasyBackups/cryps: Permision denied os error 13)
I am trying to push the backup to my NAS (using the credentials of the root user on the NAS).
The filesystem type on my NAS is Ext4.
The contents of the tmp folder seems to be owned by root:
start9@yummy-clams:/media/embassy/tmp$ ls -la
total 0
drwxr-xr-x 8 root root 160 Jan 11 06:47 .
drwxr-xr-x 1 root root 100 Jan 11 06:29 ..
drwxr-xr-x 2 root root 40 Jan 11 06:34 C6XM6Q54OYMEUZBKO56FR7FLRIJLBIEBSM37QS7H4FBEY6JLM2NA
drwxr-xr-x 2 root root 40 Jan 11 06:29 E3RNS3F3QI4CPNNQVCSNYORWLN7Q2ENU3BZWRJXFYL6JHCDWHRRQ
drwxr-xr-x 2 root root 40 Jan 11 06:38 LFRBUAQ4YUMNDKUXSWTM64SAVIZLJD2QPDW75LG7V63JJAPAM73Q
drwxr-xr-x 2 root root 40 Jan 11 06:47 QKMZAUSABU7F73YE6VFBHTHQLZYBLJDNSFT3CCQYLLNQNS2OTGQQ
drwxr-xr-x 2 root root 40 Jan 11 06:36 QW3NRKJBWPT7WRI7S2QFH5DAWZC2I4S6TTA2BA6JMT5FUHJI6XBA
drwxr-xr-x 2 root root 40 Jan 11 06:43 U3NCBKPZR75QPE7AC4QUH6NYNGP5J4DZBPNOWG5G3A5FHPGKTR5Q
Is the backup executing as start9 user via the GUI perhaps?