Same here, happens every so often out of the blue. The only thing that works is to power the watch off completely and switch it back on. Very annoying.
It should be noted that most apps in the Appstore are not from Garmin, but are mostly written by private individuals. Garmin provides a so-called SDK (Software Developer Kit) for download. This allows anyone with programming skills to develop apps.
A very important factor is the fact that the installed firmware on the watch must be compatible with the version of the SDK used by the app developer. This means that if you perform a firmware update on the watch and apps on the watch are not compatible with this FW, the respective app will "crash" ( error message IQ!).
So the developer is always "forced" to download the latest version of the SDK and recompile his app with this new SDK and then make it available as an update in the store. This is a constant process that is necessary to keep the app running without problems in the long run.
If the IQ! error message appears, the best way is to contact the developer of the respective app and ask if he has used the latest SDK. If not, he should recompile the app with the current SDK and provide the update. Of course, there may be other app-internal bugs as well. You can clarify that with the developer. In the Connect IQ App and in the Web Store there is a corresponding button "contact Developer". Many developers are from the English-speaking world. This should be taken into account.
First, you can also try to uninstall the watchface, restart the watch and then reinstall the watchface. Sometimes this helps. Often but unfortunately not. Try it out.
