PBS currently allows the backup space usage to go right up to the quota limit and run out. When this happens, it can't even prune & garbage collect. PBS needs to set a reserve for itself or allow a separate configurable limit.
Well... you might put this as a feature request on https://bugzilla.proxmox.com - if it is not registered as a problem already. This way some developers might spot it.
I am just a (home-) user, but my expectation is that a "disk full"-event should not kill the workflow and cleaning up should be possible...
Unfortunately nothing you can do yourself at this moment. You have to get in touch with tuxis, they can fix it for you.
I'm in the same spot, a bug report discussing this is here: https://bugzilla.proxmox.com/show_bug.cgi?id=5376