When things get funky down in the asp.net temporary files below the Windows folder, the only thing I know to really solve it is to delete the temporary files and folder so the asp.net compiler can take another clean pass at it.
However the files cannot be deleted while in use so you have to stop the IIS web site, delete the files and then start the web site again.
Before doing that however, I suggest tell them set 32 bit enabled back to false, it is not needed by mojoPortal and at least one other person has had similar error that resurfaces again within a few days if 32 bit is enabled even though setting that first seemed to fix the problem.
I think changing a setting like that on the app pool sometimes seems to solve an error like this because it forces a recompile and sometimes that fixes it at least for a while but some funky file may remain and deleting them all is the only sure way to solve it. So that setting is really not the cause nor solution but the support guys at Arvixe may think it is since it sometimes seems to fix similar errors, its really the recompile by asp.net compiler that fixes it and that is merely triggered by toggling that setting to a different value.
so, first set enable 32 bit to false.
then check the error in case that setting may have changed the path to the temp files
stop the IIS website
delete the temp files and folder referenced by the error
restart the IIS website and make a browser request for the home page or setup page
the startup compilation may take a few minutes after that but it should fix it, then click around a few pages to warm it up and it should be working as normal
Hope that helps,