Hi,
It depends on your hosting ; you should see that with them.
Otherwise you can see to set the "error reporting level" to "maximum" in your Joomla global configuration ; it could help and display the error messages instead of that "internal server error" page.
But without any information about the PHP error ; it will be more than complicated to know what the issue really is (and so, to fix it).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.