Very weird issue on one of my client sites running WPMS 3.0.1 - it was previously WPMU.
Clicking 'Switch to' on any of the users (in user manager or profile editor) it 'switches' but it says it's switched to my user acct from my acct.
Anyways, it looks like it boiled down to COOKIEHASH being empty. I checked why it was empty and it was my wp_sitemeta table not containing a record for siteurl - which is very weird. Why is it very weird? Because new installs have this option. It *may* be something to do with the upgrade process itself in WP from WPMU or something entirely different.
Figured I'd leave this post here for anyone to read if it helps them.
To fix this issue:
- Add siteurl value to your wp_sitemeta table (set one up for each of the sites based on site_id)
- Clear your cookies for the site
- Login again as your admin
- Go to users menu pages and switch to user as normal