I would try sending the signature capture data with AsyncHttp or Net::HTTP. Also, make sure your ET-1 has the latest firmware, as older firmwares had a certificate issue where it didn't have any root CAs installed, which caused every site to be untrusted. You could work around this if you were using Net::HTTP, because you could set the verification to VERIFY_NONE, but the other HTTP stuff in RhoElements would not work.
Also, I've had good luck using jQuery Signature Pad instead of the ET-1 signature capture API. Of course, it is a lot more basic, so you have to do something with the data on your own, but in my application, that makes more sense.