ok, in this case, please leave some screenshot of the issue.
I literally get a blank white page with the words "Invalid User ID" displayed
Wordpress DEBUG to a LOG could help also.
We didn't get any information from you. You even didn't tried the topic suggested by Sofy. You don't see any relation but this does have some.
When I have WPforo activated I am unable to edit WP user permissions. I get a Invalid User ID error on updating a user. As soon as I deactivate WPforo the issue is not there.
Please also specify how you do this and what plugin you use.
What information can I give you ? I didn't understand the topic suggested as it did not mention anything about the issue I am having, hence saying it was not relevant.
If the plugin is activated and I try to change any user privileges, like make someone an admin for example, I simply get a white screen with the words Invalid User ID. It does not save the settings. I disabled all plugins and loaded them one by one. As soon as I activated WPforo, the problem was there, so when I now need to update user settings, I have to disable Wpforo first and then activate it afterwards.
Does that make more sense ?