Bitcoin Core RPC "importaddress" for a P2SH watch-only
I believe the issue is that you forgot the label parameter. According to the RPC help documentation, the parameters for importaddress should be:

 importaddress "address" "label" rescan p2sh


You have used "address", rescan and p2sh but not label, so the error is likely due to an issue converting the parameters you provided into the ones it expected. 

But the fact that the error message is so hard to understand / unrelated to the actual error is a bug, please open an issue on github here: https://github.com/bitcoin/bitcoin/issues