Steps to replicate the issue:
Actually it is difficult to reproduce. It seems to depend on the environment and even the user context.

What happens?:
When loading "Special:FormEdit/MyForm/MyPage" with a form that contains a "combobox" input, there are JavaScript errors like

ReferenceError: pageforms is not defined
TypeError: pf.ComboBoxInput is not a constructor

and the form does not get loaded.

What should have happened instead?:
The form should get loaded

Software version: PageFormd 5.7.2

Other information: As mentioned above, the error is difficult to reproduce. But from looking at the extension.json I feel like an explicit dependency of ext.pageforms.ooui.combobox to ext.pageforms may be missing. Other similar modules (ext.pageforms.ooui.autocomplete, ext.pageforms.select2, ...) do have this dependency declared.