>What are oApp.Occurs() and oApp.At() ? Are you sure it is the UCase() rather than these which is causing the problem?

Same as VFP Occurs() and VFP At().

The last line of the Stack trace gives this:

at Microsoft.VisualBasic.Strings.UCase(String Value)

>IAC, it would be more efficient to create upper case versions of the string for use in those functions rather than repeating the conversion each time.

Will do.

>Does using ToUpper() rather than the VisualBasic namespace UCase() also cause the OOM ?

I never use that as all functions are mainly from the framework. So, if we change environment and do a framework for that, all the client code would remain as is for those functions in case ToUpper() wouldn't exist. But, at this point, it is difficult to try a few changes as it is only in the production that the problem started yesterday. That code as always worked well for a few years and still works well in our test servers.