I think it is likely the error is not on the Bitcoin side it is rather on the mempool. To be sure, try to add the debug configurations on my post above, I believe it will give clarity why the mempool is returning HTTP 500 since it will log more output on both electrs and mempool.
You are right.
Inspecting the URL via the Browser dev tools console gets me the same HTTP 500 error
There may also be an access error. Start a bitcoin node with the
-debug parameter and inspect what the node writes at the time of the request. You can use without -daemon.