It may be a "security feature". Many sites prevent themselves from being "framed". If you search this, you will find it's not a Web Direct, Web viewer issue:
'google map cannot be viewed in a frame'
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Adding the flag '&output=embed' to the Google Map URL did he job.
Awesome! You should mark your own answer, Martin, as "correct". It's how to fix the issue.
I just pushed you to possibilities.