What browser are you using? If you use Chrome, that’s likely to force http to become https, and as a result it doesn’t work with local IPs due to mixed content. You can try Firefox or Safari which doesn’t force https.
If you have OpenSprinkler 3.x you may want to upgrade your firmware to firmware 2.2.0 (we haven’t officially released it yet but it’s going to be officially released very quickly). With firmware 2.2.0, remote control can be done through OTC token and secure OpenThings cloud server, so that way https://ui.opensprinkler.com would work fine with it.