Edit Customer => Error 500

[Lukas Truninger]

Hello,
If I like to edit a customer, allmost all customers endup on the error 500 page. However, I found one customer which opens correctly in the customer edit window. For debuging I have then edited directly in the database next customer (which ends up on error 500 page), using exactly the same values for all fields (except the ID) in order to check if the issue is caused by some illegal values. This also includes the associated address. But it does not solve the issue. I have now 2 identically customers in the database: one I can edit, the other ends on error 500 ....(respectively on attached page if Debug Mode is on).....??
I already spent a whole week to investigate this terrible issue, I have deleted cache, I have deleted /var/cache/prod, I have deleted & rebuild htaccess, I have set server max_vars to 9999, I have set max_execution time to 18000,  .......????
Please see this video: https://prostage.eu/customer_edit.mov

PS 1.7.6.1
Thanx for any help
Lukas