timtecsa
Hi Tim,
Also check if you have any security extension on site kindly disable it, sometime the extension does not allow other users to login on admin from outside country, depend on configuration you applied.
And for the 500 error, in most cases it comes when the current template is heavily customized it does not able to override the template due to heavy conflict, you can compare the files in JAEM and restore the file without custom work first by taking backup of files.
Let us know once your login issue is fixed. We are able to check it better.
Regards
