Error Message: Could not find file with specified id: 7c669cdf-d211-4f2a-9d2c-e6e7ec7f715e

Hi all, I need urgent support - I just completed translating a huge file yesterday and today I reopened it, made a minor change and then the file got reviewed and signed off (by myself, I am using the freelance version of SDL Trados Studio). When I wanted to "save target as" the above mentioned error messages appears. Never happened before. I closed all the programs, shut down my computer, and tried again, but the same message appears. What can I do? I am desperate. I have lost so much time trying to find an answer in the community and knowledge base and also to only figuring out how to ask a question on this forum, please help asap! Thanks in advance. Selina