500 internal server error appears when your script requires more than allotted physical resources on the server to run successfully usually happens in poorly coded scripts designed by inexperienced developers or by installing open source scripts which basically require higher resources to run.
There are several ways of resolving it.
1> Deload the script a little, try disable some of the widgets/plugins/extensions. This can resolve your issue.
2> If you are a reseller, you can shift that particular domain as a normal Linux shared hosting package by purchasing an individual shared hosting package for this particular domain, in which case we will double the physical allocations of this domain as per the requirement, similarly it can be tripled, or quadrapled by purchasing multiple shared hosting packages for same domain. Domain will still remain in your control, just that its resources are increased.
3> If there are multiple accounts facing the issue, you can pay double the price of reseller hosting package active in your account, and we will double the resource allocation of any domain in your account as per your demand when such errors appear. You can create an Add Funds invoice from your http://whitelabelhost.in account and make payment for it, and reply us here with the invoice ID.
4> You can purchase a VPS /Dedicated server for it, ultimately, if the requirement of your script is very very high, and its not suitable to be run within shared hosting environment, you can try to purchase a VPS from us and shift your this particular domain over there, depending on your budget and your client's requirement, we have several packages available.
Let us know how you would like to go ahead.