Store works fine but admin page give HTTP ERROR 500

[Arunachala]

Just upgraded to zen cart 1.5.5e. After thorough testing and making sure that everything is working fine replaced the live store with the updated store. Store is receiving orders just fine but suddenly the admin page gives HTTP ERROR 500. Of course the admin was working for me to make changes. The store is at www.bookstore.sriramanamaharshi.org. The admin is at /NamoRamana12. This is driving me crazy because the tech support at Bluehost is unable to find anything, no error log message or zen cart debug message. I know it is not easy to find the cause of this but any direction of where to look for answers from experts will help.Thanks