Here’s a reply from the extension creator:
That’s a bug in SketchUp’s WebDialog routines, not in FlatText. It appears to be having trouble creating the dialog window from our HTML file.
One possible workaround would be to make a copy of a FlatText and revise (not edit) the copy. That command uses the standard dialog box, so it avoids the Web Dialog issue. (Edit uses the same WebDialog that Place uses.)
For larger text blocks, you can create a text file and copy-paste between it and the revise dialog.
Sorry this is giving you trouble - it’s an issue between SketchUp and Windows, so it’s out of our control. If enough people complain, maybe Trimble will fix it, or at least tell us developers how to work around it.
By the way, are you using the latest version of FlatText? We’ve fixed a few bugs recently. I don’t think that would fix this issue, but you never know…