I presume by your reply that you did do all those steps exactly in that order. If not, do them.
Is the main server used for any other system/application?
Are there users who may log into the main server?
Do you have only ONE, repeat that, ONE instance of WinCC running on the server?
If there are unknown shares, conflicting applications or users active on the system when you are trying to download, run, or do other things on the Server, it can behave erratically, make you think you are crazy, and convince you the software is terrible.
And lastly, is this a new installation, or is this a server that has been running for some time, and now has problems?
If it's been running for some time and now has problems, what changed?
(I'm not implying you did anything to change it)
Were there any network events that may have preceeded it?