OK after more than 30 hours of hard work and lot´s of testing I think I got it 🙂
It was complicated but these pluginns was involved (I think):
When I was NOT logged in: “Zamango Page Navigation” (This plugin adds an advanced page navigation to WordPress)
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And when I was Looged in: “Youtube with Style” (Show You Tube videos with a custom stylish player, also create galleries, how user videos in widget form, and search in real time via the youtube api)
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And sometimes I had problems with: “add widgets to page” (Adds Wiget space to Posts and Pages using shortcode)
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After deactivation of these plugins I got it to work. Lately I activated “Zamango Page Navigation” again, and the forum is still working fine in Explorer??? But I´m happy, because it looks like I soon a ready to use Simple:Press instead of PHPBB 🙂
A little P.S.:
I had to give up regarding “Editor (TinyMCE – Rich Text)” – I simply can´t get it to work in… Explorer 🙁