So after some clarifying information from a chat transcript, what you are actually trying to do is merge Data Explorer accounts. Our contact us form simply doesn't work for those accounts. I'm not sure why it throws a server error with them, but it's very much by-design that you wouldn't be able to use that form to merge those accounts anyways, so the fix would just be a more graceful error message, which is probably not the goal you wanted to achieve here.
Data Explorer uses open source software and a completely different accounts system, which does not support having more than one credential on your account anyways. Logging in via a bunch of different ways will continue to create a bunch of different accounts for each different login method.
To be quite honest, you could probably get them merged by bugging a dev to merge them, assuming there's actually good reason for the merge (e.g. not a bunch of empty accounts that have never done anything). But merging SEDE accounts is not something we officially support, simply because with the way SEDE is built, it doesn't make much sense to support merging accounts there outright. Merging may move any queries you've created, but you'd still end up with one credential, and you'd still continue creating additional accounts by using other credentials. You'll just have to remember which one you used for Data Explorer and keep using it.