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 "Transfer" or "Platform to Platform" tools to create a "Batch" instead. Doing this will avoid you to have 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 got this issue while importing a file (.txt,.csv, etc...), you should have to split your file into smaller parts first. Once done, you will be able to import them one by one correctly and then, merge them on the destination platform.