I calculated with 2,5MBit / 10 -> 0,25MByte to take in account all overheads.
But you are right. To be safe I should set the asynchronous- upload-limit to 1 - until we have a better upload speed (hope so before I die).
In this Link I found a comment from JonMikeIV that the standard value for timeout is 10 minutes. But I guess he was wrong?
My big thanks to you for the fast and competent help!