Hello Dan M,
The exception you found is handled inside the tile cache subsystem of RadMap, and you shouldn't worry about it.
It is thrown when the MaxExpirationTime is not specified. If you want to avoid this exception, then you should specify MaxExpirationTime property.
Sample code is below.
FileSystemCache).MaxExpirationTime = TimeSpan.FromDays(15);
All the best,
the Telerik team
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