UPDATE: We never determined the root cause of this problem and it never surface again.
We notice during implementation of a development site that suddenly, two users are unable to access a site collection. Those accounts can authenticate to the main site, but when trying to access a particular site collection, they just get a blank screen. No errors displayed, just a white blank page.
We log in as a site collection admin and try to add one of those users as a site admin and this time, upon pressing "OK", we get this message:
Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION))
We spent some time researching this and unfortunately, didn’t come up with anything useful. There were some messages in the diagnostic log, but it was hard to exactly correlate them with this issue.
In the end, we deleting the site collection and re-created it and that solved it.
If I figure out what caused this in future, I’ll update this post.