I enabled Joomla debug, and error level reporting to maximum.
Nothing changed in the error page. (no debug information)
Discovered something super-weird... if I change the product title, e.g. add a blank space at the beginning, the product works fine!
If I change it back to the original, it crashes again.
Here's the original product title: Φορτιστής, συντηρητής και αποθειωτής συσσωρευτών 2 θέσεων φόρτισης NOCO GENIUS2X2 4Α (2Α/θέση φόρτ)