LoTW returns "invalid adif file header"

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LoTW returns "invalid adif file header"

If I use the menu /QSL/LoTW/Import/Download and process data from LoTW website the following message occurs:
"Something is wrong because LoTW server returned invalid adif file header. Yout LoTW username/password could be wrong or LoTW server is down.
Do you want to show the file?"
My access credentials are definitely right and the LoTW server isn't down.
But if I check the returned file from LoTW, I see that this isn't an ADIF file.
So the message "invalid adif file header" seems to be right, because of the non-ADIF-file.
Are there any suggestions to solve my problem?
73 de Jens