I lost count how many modules we have installed and how many more we did. but we have heavy customized.

However, we have no other errors and any conflicts we may get, we can usually resolve as they come.
But we are completely newbies dealing with paypal api, so, we are really stuck with this one

what modules could be causing this ?

I have enabled currency: USD, CAN, GBP, AUD, JPY, EUR, SEK, CHF, FRF

ZC paypal module set to : " Selected Currency"

about the error logs, it's strange but I could not see a single mention of 500 internal server error pages in the logs.

I could see some error logs with the godaddy seal contribution , so to be sure I have deleted all the files on that contribution, but seems to be no related.

I don't know what else to do.