X15 2250SN fault code 1911

[MPDiagnostics]

From what you are describing in your post it seems to me you might have high return line restriction in your return line(s)

You mentioned that you did the return flow tests which is good, however I believe quickserve also lists a test for “drain line restriction”. Put a gauge on it and try that test under different conditions.
Check for kinked drain lines and test the return line for any debris stuck in there.