how to catch 500 internal server in hosting machine

HI..
I have my webapp which its run fine, the same web app i hosted in my hosting provider everything works fine suddenly some times it give me this error

I have place this code in my webconfig file too
<customErrors mode="Off"/>
Still its shows the same message
500- Internal Server Error as in the above pic..
how to catch what the exect error is
Thank you 
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