I'm running v5.51 now but it would give me the error saying the url needs to have http:// and wouldn't let me save the project. I've had that error before if the #file_links file was empty and didn't have any url's in there but this is not the case this time. When I switched back to v5.51 everything works again without making any changes.
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When I use the test button the URL field is blank.
Rolled back to v5.51 and all is good.