You could get this 199 error if the current process you have launched is reaching timeout (probably because it's a quite large playlist or list).
We recommend you to use "Transfer" or "Platform to Platform" tools to creating a "Batch" instead. Doing this will avoid you to have this error message; by letting all the process running in background on our server without having to deal with browser timeout issue.
If you got this issue while importing a file (.txt,.csv etc...), you should have to split your file in smaller parts first. Once done, you will be able to import them one by one correctly and then, merging them on destination platform.