I am posting this to share my solution to a problem (and also for google's sake) I came accross today, namely an "Internal Server Error" message in Silverstripe's backend when trying to access a (so far working) UserDefinedForm Page.
As I was running php 5.3 in cgi mode (or in other words switched from the default php 5.2 do 5.3 via .htaccess), the server's R limit for processing time of 6 seconds has been reached. For some reason this specific page took too long to be processed and hence the message.
I could not take a look at the server error logs, so I talked to my webhost and since I have own server, they found the problem and managed to increase this value to 12 seconds. I am not sure if that is applicable on regular shared hosts, though.
I am planning to switch to php 5.3 or 5.4 in near future since this can not be a viable long-term solution.
I hope someone can find this of use.
Edit: Silverstripe version is 3.1 beta and UserForms is the corresponding module version.