That results stack trace looks similar to Backups fail with error: Object reference not set to an instance of an object except that yours started in TestFilter while the other started in Backup. They ended in the same way, which is possibly a failure in collecting the output lines, but my previous attempts to debug the failure dead-ended. Interestingly both failures are Linux. Sometimes updating to latest mono can solve surprising things. [Solved] Failed to connect: Only http or https scheme is allowed in 2.0.4.10_canary_2018-12-29 is the latest case where I wished I’d suggested trying it. I’m not sure if you want to try updating mono though.
If you happen to have a Windows system showing a stack trace like that, my chances of debug get higher. You could also file an issue for the developers, especially if you can come up with a very simple test case.
An alternative suggestion is to try real shell instead of GUI one, being careful to quote strings as necessary. While I’d like to solve the Object reference failure, I think I’ll have better luck at getting test-filters running. Following your steps didn’t break Commandline (but my /root is almost certainly smaller – does it matter?) however one oddity is that adding the cache exclude broke the Duplicati exclude. Under shell, it still works:
# duplicati-cli test-filters /root/ --exclude="/*root/.config/Duplicati*/" --exclude="*/.cache" | grep Duplicati
Excluding path due to filter: /root/.config/Duplicati/ => (/*root/.config/Duplicati*/) || (*/.cache)
#
# duplicati-cli test-filters /root/ --exclude="*/.cache" --exclude="/*root/.config/Duplicati*/" | grep Duplicati
Excluding path due to filter: /root/.config/Duplicati/ => (*/.cache) || (/*root/.config/Duplicati*/)
#
I haven’t yet tried reversing order in the GUI. I wonder if this unusual case of repeating options won’t work? OK, I had to try reversing, and it seems like perhaps the last --exclude option it gets is the only one it does. Seems like a possible bug (although a somewhat obscure one) in however Commandline is implemented. For the clear bug of NullReferenceException, does it also fail with short output e.g. /root/.config/ not /root/?