LPU file is opened read-only

As we are splitting a big software project into multiple parts, we also want to split our translations.

I copied all the files to a new location. When opening the lpu file, I cannot "create/update string list" because the whole file is opened in read-only mode. The file itself and all other files in this folder and the subfolders are not read-only from the windows file explorer perspective.

I'm a beginner with Passolo. We are using Passolo 2015. Anything more you need to know to be able to help me?