Hi
I'm not being redirected to error 500, but see a blank page which after forcing errors to show pointed to an invoice creation fatal error. I believe a function creating pdf has problems with Your language special chars.
Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 122880 bytes) in /var/www/vhosts/creafesta.cat/httpdocs/libraries/tcpdf/tcpdf.php on line 15688
Try maybe reinstall tcpdf library. Check how to force it to use Your language characters and ask on VM forum about this issue. Unfortunately I'm not able to help since it's not connected with our product