I would look and see if anything is logged in the mojoPortal log whe you see the blank page. First clear the log to get rid of old errors - Administration Menu > System Log
Removing that from your Web.config is not going to make you any more secure.
The security benefit to enforcing medium trust is mainly because you are in shared hosting and it can prevent other .NET apps from other users from doing bad things that could impact your site. Medium trust limits what the code can do so really its limiting your own app, like you don't get the benefits of using NeatUpload. It limits code from doing things that could be used maliciously but it also limits non-malicious thigns that could be done but require more trust. So the idea of enforcing medium trust is to protect your site since it is on the same server as other sites, if they were really enforcing medium trust then other sites on the server have less possibility to hurt your site, but if they have this workaround where you can change it in Web.config that means they really are not enforcing it, others can do the same thing and elevate their site to full trust and maybe write code that could attack other sites on the same server.
So removing this from your web.config is only giving you the disadvantages of medium trust without any benefits because they are not really enforcing it if anyone can work around it in Web.config. That means the bad guys could configure their sites with malicious code to run in fuill trust while your site which has no malicious code is running in medium trust.
If you were on a dedicated server with no sites but your own there would be nearly 0 benefit to running in medium trust and in fact it would be preferrable to run in full trust so the nice features like NeatUpload can be used, its really designed to make shared hosting more safe by protecting you from other code running on the server that is not yours, the tradeoff is it also limits your own code. I don't get a good feeling about your host, it seems they really do not understand what they are doing and have some weird misconfigured trust policy.
Hope it helps,