Help Math on screen Symbols are shown incorrect

[Alexander Gahr]

Please i ask this question now a second time, because nobody replayed  
to my first mail.
Everything seemed to work fine but after installing a new latex  
package, everytime i write a
math symbol they all appear wrong.

If i try to write a Sum Sign with \sum there appears a blue P par  
example.

In the final PDF everything is right again.

Please help i can't write formulas in LYx anymore.

Alex