Yes, I was going to suggest it would most likely be related to the piping of temporary images. You’ll note that the relevant details are described in the implementation section. By default, these will be written to /tmp on any OS other than Windows. Also check whether that location contains obsolete temporary files left behind by failed MRtrix3 commands:
ls /tmp/mtrix-tmp-*`
and remove them if you find any (assuming there are no jobs currently running…). This might explain why your commands are failing in the first place (no space left on /tmp). If /tmp is genuinely too small, try setting the TmpFileDir option in the config file to e.g. . (current folder), as suggested by @ThijsDhollander (instructions here).
I do however note that the documentation for the TmpFileDir config option doesn’t match the current implementation – but that’ll be fixed shortly.