Support

Please go to your site's Global Configuration page and check the Path to Temp Folder. If it's /tmp you have to change it. Otherwise go to your site's backend, click on the System, System Information menu item and then on the Folder Permissions tab. Scroll all the way to the bottom and check the badge to the right of the "(Temp folder)" indication. Does it say Writeable?

So your temp-path is not /tmp, it's /[server]/[account]/public_html/[domain]/tmp and doesn't need to change :) Since the folder is writeable we have to see why saving data there doesn't work. Do you have enough space on your server?

The error message literally tells us that at this point the server's operating system responds that no data can be written to the temporary directory. We established that directory exists and is writeable and you have enough space. So the only part we are sure about the error is that it comes from the host.

I suggest talking to Bluehost about it. It's strange that sometimes you can write to it and sometimes you can't. Do they have an I/O per second limit? That would explain why it happens randomly and sometimes: if the server's a bit busy and you're backing up lots of files you might end up going over the input/output operations per second limit.

If that's a thing we can make the backup slower (throttle it) to overcome the problem. We could go to the Configuration page and enter
Minimum execution time: 5 seconds
Maximum execution time: 3 seconds (yes, max is LESS than min, it's not a typo!)
Execution time bias: 50%
That's a 50% duty cycle which should work around that issue.