After uploading a file with a comma "," in the file name, the requested document URN is always null. Thus, the file is re-uploaded every time I try to open the file. Is this a bug or is there a naming convention forbidding commas in file names?
The problem was in my code. I splitted the response using commas, thus when the response contained a file name with a comma, the splitting was different which caused the problem. Thanks to @Jaime Rosales for supporting!