Hi @John Kanost ,
I'm glad to hear you solve the problem, if you have any issue about SharePoint, you are welcome to raise a ticket in this forum.
I would make a brief summary of this thread:
Issue Symptom:
Unusual Document Upload error when replacing existing documents in SharePoint 2019 on-prem.Once this occurs, the entire document library toolbar becomes locked and the only way to take any further action is to refresh the page or navigate away and back to it. It almost seems as if the normal prompt (i.e. replace or create a copy) when uploading a doc that already exists is failing for some reason.
Current status:
have to install KB 5002580 together with kb5002538. Download and install kb5002538 solved the issue.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!