You could get this 199 error if the current process you have launched is reaching timeout (probably because it's quite a large playlist or list).
We recommend you use the "Transfer" tool to create a "Batch" instead (paid plans only). Doing this will prevent you from having this error message; by letting all the processes run in the background on our server without having to deal with browser timeout issues.
If you encounter this issue while importing a file (.txt,.csv, etc.), you should first split the file into smaller parts. Once you do so, you will be able to import the parts one by one correctly and then merge them on the destination platform.