Yes, I've had the same experience, and it wasn't any fun......But here's the only workaround I've been able to accomplish:

1)Find a very very good CSD on the phone,
2)Explain what you're trying to accomplish,
3)Let them call web support themselves (who will likely tell them that e-certs and H fares can't be combined),
4)Hope that the CSD knows this to be "nonsense",
5)Ask them if they will allow you to purchase the ticket on united.com USING the e-cert while they wait on the phone,
6)Have them 're-fare' the tix up to H by charging the difference to your CC, and,
7)Thank them profusely.

Its not an easy thing to get them to do, and they may tell you to take a flying leap, but I've gotten it to work in the past. YMMV.
Best wishes, Dave