It is a not uncommon error. You can Google the assert error message or key parts of it to see lots of similar reports in the past, and recently.
Start by trying to trash prefs. (use Peter Gates PT Prefs Tool
http://petegates.com/pg-pt-prefs.html). If unsure trash everything.
If that is not it, in this case I'd suspect (=guess) file permission/ownership issues.We might get lucky and something might be obvious... run the following commands in Terminal.app. Copy and paste the text output here (no screen shots).
cd <directory where the loop files are>
ls -l * # just need a few lines showing some files that had problems.
id # who you are.
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Are there any other problems happening?
Other things to try (after trashing prefs):
Test for this problem with a new empty session
Create a new macOS admin account, and try doing the same thing there with a new empty session.
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A few recent reports of this, but folks doing different things.. will be interesting to see if this is a problem specific to 2019.5, or just the same old same-old.