Hi,
Start with Basic Troubleshooting to find out the error details.
Without knowing the error details my best guess is an issue with the .NET framework version. Site Membership Pro has packages for .NET 4.5, 4.0 and 3.5 and using the wrong one could definitely crash the site.
Since you are using mojoPortal 2.4.0.3 you definitely could not use the 4.5 .NET package of Site Membership Pro, since mojoPortal 2.4.0.3 is compiled against .NET 4. Your hosting may be .NET 4.5 but your mojoPortal installation for version 2.4.0.3 is compiled for .NET 4.0. So most likely you need to use the 4.0 package of Site Membership Pro.
Hope that helps,
Joe