I'm having the exact same issue. Got my iPhone 6 unlocked from AT&T, bought a SIM card kit from Boost, and now IF the website lets me log in, and by some miracle lets me enter the IMEI/MEID the ICCID always fails. Either with the error message OP posted or just simply, "We had trouble processing your ICCID. Please try again".
I entered my IMEI on the page where it asks to see whether or not it's compatible and it said it was, so I went ahead and bought the SIM card kit.
Would you mind sending us a private message with your device IMEI and SIM card number? We'll be glad to check on it asap.
Hi there! Sorry for the delayed response. Support is available from 6:00 a.m. to 10:00 p.m. CST. We would like to look into this for you. Please send us a private message with the details so we can assist you. In order to send us a private message, you will need to click on one of the agents profile (they will marked as such), click "send this user a private message".
Please send us a message with your phone number, PIN code and the IMEI number of the device you are trying to swap on your account.