Is this always reproducible? I have never seen this error message before. Definitely a problem that should be reported to Support at https://gateway.sdl.com/. With the help of the project file (LPU) and the SDLXLIFF file, Support would be able to analyze this and help if necessary.

Hello Achim Herrmann, 

Please advice me how to generate a support request. 

The displayed message is showing that your company has no support contract. The forum is then the only way to ask technical questions about the product. However, in many cases it cannot replace support (with contract) and does not offer the possibility of uploading files and projects for analysis. As the owner of the forum, I try to answer questions of general interest in my spare time. The more time-consuming analysis of specific customer problems is reserved for my colleagues in the support department.