Magento Error fix

We did setup a new magento cart for a new project. We did some changes and did add extensions. 1. Now we have a problem and cannot edit customers anymore. We get a blank page. This need to be fixed. 2. We get an Error 500 if we try to login in the admin panel. After refresh (F5) 4-5 times it works. Also this need to be fixed. Website is hosted on 1and1 server. After all that I think we would need some more help. Thanks in advance