Attached difference file to document the above issue with scaling. It shows that the opengl output mode is unavailable, so that is a probable reason why full screen scaling is unavailable. If the opengl mode code was introduced, then it should be possible to scale full screen. As it is, the output mode is necessarily reverting to surface. Although surface should also scale to full screen, I've confirmed that opengl mode is expected to work where surface is not (in a private dosbox build; this does not refer to vanilla dosbox).
Also, confirmed that dosbox-x uses a new text mode which stretches the horizontal width of the screen beyond the default width (640). So, where using the scalers such as normal2x, the window will stretch beyond 1280 pixels in width. If the default dosbox text mode was used, then the 2x scaling should fit within the screen space of a 1280H monitor and not generate an error while switching to full screen with a scaler active.
I would first add the opengl mode and test full screen scaling, if there is any issue, then revert back to the default text mode of dosbox. 😀 Otherwise, the window scaling (non-fullscreen) works with the "forced" parameter, so this is a simple workaround.
Edit: removed attached diff file and replaced it with a patch attached to the next post in this thread.