Here is the answer from Siteground:
I have checked the case and was able to reproduce the issue but wasn’t able to resolve it. I tested the writing to the /tmp folder with the suggested tests in the article you provided and writing to a temporary file is possible as you can see from the test script:
https://kafeinsiz.com/tmptest.php (i will delete this file in 3 days)
I also tried to locate if there is an option within the plugin to set the temporary directory but didn’t find such an option. I recommend that you either reinstall the plugin or contact the plugin’s developers in order to ask them what may be causing the issue as the tmp on the server is writable and there is enough room as well.
