Thanks, good to know. This problem seems to come and go depending on perhaps the load of the SmartThings platform at the time you load the dashboard or any other mystery factors. It is good to know that you were able to resolve it by reducing the number of devices. For many that is not an option and not all who have tried it have been successful.
SmartThings support has heard from a growing number of people affected by this, but now going on 3 or 4 months it seems unlikely to be resolved with any urgency. I’ve tried several times to get ady to take a look at this in case there is something that could be done differently to work with devices in webCoRE, but as far as I can see whether the devices will load or not is at the whim of the platform.
An option that may be helpful depending on what you need to do with the dashboard is to skip the loading of devices entirely. If the dashboard loads you would not be able to select devices for adding/editing pistons and devices in existing pistons would appear as the raw device ID rather than name, but you would be able to do some device-agnostic edits, see your pistons and tiles, and export your pistons to a backup file. I cannot recommend that anyone actually does this and will not post the code publicly because running webCoRE without the device data is untested and it would probably be quite easy to make the situation worse.