Can't access "customize" pages

Hi guys, I am running Catch Base Pro on rt-erfurt.net and as of yesterday, I am expiriencing problems I never experienced before. First: after inserting a new image into the slider, my entire customized slider was gone and automatically replaced by the default slider. Second: As soon as I click "Appearances" ---> "Customize" or any other option under "Appearances", I am landing on a blank page: Looks like that I can't acess any setting page anymore. I am lost. What happened? Please help! Thanks community! Josef