user control is ambiguous in the namespace ‘ASP’

This one was a particular pain.

Long story short try this in your web config

Set batch=”false” under compilation

<system.web>
<compilation debug="false" batch="false"></compilation>
</system.web>

<system.web>

</system.web>

This will allow you to drill further into the problem, its not a particularly desirable attribute to set in production.

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