In the meanwhile we're using Strato on a regular basis, but haven't seen this 0x80070091 message before.
But if you want to check to what extent the copy to an online storage was successful or not, there are several options:
1) Mount the online storage as a drive again and use a tool that can compare a directory and all its subdirectories and files. For example, you may use a (free) Linux like "diff" command under Windows:
diff --brief -r
These 3 lines are one command line, of course.
The --brief option (note: two times a '-' ) says: only tell me which files differ or are present in one dirname, but not in the other.
The -r option (note: only one '-' ) says: traverse the directory and all it's subdirectories.
When you execute this command it will tell you which files are different ( incomplete upload ) or which file or directory is present in one directory hierarchy, but not in the other directory hierarchy ( not copied / uploaded at all ).
2) Another option is to use a (free) Linux like rsync tool under Windows if you have a paid Strato subscription.
rsync -rvt --progress -e "ssh"
Above-mentioned 3 rsync lines are one command line and again, note the single '-' and double '--' command options.
The rsync command will show you which directories and / or files are not entirely copied _and_ will complete the copy process if necessary.
Note also that the destination path is sometimes different, depending on the Strato access method you're using, smb/cifs access has an extra "root" as part of a directory path, see:
The advantage of rsync is, it will always complete interrupted transfers. Actually, we do not use copy commands anymore, only rsync.
In addition don't worry too much in case you do not want to think about what you've changed or not in a particular directory tree, just use a high-level directory, rsync will figure out what has been changed or not, and will copy only the changes to the destination directory.
Using rsync, speedups of a factor 1000 to 10 000 ( sometimes much more ) are quite common in our case.
3) Using Strato there are a few other scenarios, but 2) is probably the most interesting one ( delta-algorithm ).
Hope this solves your problem.