Ah, when an Anvil message says to check the browser console, it means to check the Javascript console on the browser. That internal error means something happened in Javascript that Anvil doesn’t know how to handle, and it’s suggesting you investigate there.
Every browser has a different method to open the browser console. Look up how to do it in your browser, then run the app to generate the error (when running in the IDE, run the app in a new tab rather than embedded in the IDE). The browser console should display a bunch of messages. Some of those will be the error that triggered the Anvil error. That can sometimes give a clue as to what’s going wrong.
Likely, though, there won’t be anything you can do to fix it, since you don’t control the generated HTML.
There’s another approach to embedding HTML in an Anvil app that’s more complicated, but should avoid the issues of incompatible HTML. It goes back to the iframe technique you originally tried.
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Write an HTTP endpoint that calls the server function and returns the HTML. Anvil Docs | Creating HTTP APIs
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In the form, have an iframe whose source is the address of that HTTP endpoint
Your map HTML is now basically in a page all its own, displayed by the iframe. It should be outside of the Anvil environment, so shouldn’t conflict with Anvil’s HTML.