We've not managed the reproduce this issue on our varying test environments with search enabled. Could you provide more details on the error you get and the exception\stack trace it produces?
The only time we did get an error similar to yours is when we mismatch the codestreams of the ExtLib and the core XPages runtime, like running the 901 ExtLib on Domino 90, or the 9.0 ExtLib on 8.5.3. This is a configuration which we don't support. Many things will continue to work but it isn't an environment that would be recommended.
But maybe we're missing something here. Could you get back to us with further details on your configuration?
Regards,
Paul.