502/503 error when creating new run from 'test executions' with > 173 tests selected

[mlongmiresqa]

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Description of problem

Creating a new testrun with more than 173 tests selected in an existing testrun causes a 502 or 503 error. I've seen both errors while reproducing this, but 502 seems more common. I created a testrun with 175 testcases on the demo site to highlight this issue. The link is in the steps.

Version or commit hash (if applicable)

This was discovered on 12.0 and is reproducible on 12.6.1 demo site.

Steps to Reproduce

1. Go to https://public.tenant.kiwitcms.org/runs/7209 or find a test run with more than 173 tests.
2. Click the checkbox to select all tests cases. Ensure there are more than 173 selected.
3. Click the hamburger icon followed by New Test Run. Observe a 502 error occurs.
4. Go back to the testrun.
5. Select 172 testcases.
6. Click the hamburger icon followed by New Test Run. Observe no error occurs and a new test run can be created.

Actual results

User is unable to create new testrun from existing testrun if more than 173 test cases are selected.

Expected results

User is able to create new testrun from existing testrun if more than 173 test cases are selected.

Additional info (Python traceback, logs, etc.)

chrome_pynCdXg5ep.mp4