Help! Bug (?) with new user Activation on multisite sites

Hi there,

Hope all is well! I am a seasoned web dev/designer, but very new to WP. We decided to use WP for building some software / app prototypes for a client. Things were working well, and we managed to set up a multisite network, add a few sites, installed some plugins, created some themes, etc.

However, we are now almost ready to start letting our clients access the sites for starting to review, and having a problem adding new users.

What I'm trying to do
- login to the main network admin, go to any of the 3 sites
- add a new user under "Users" -> "New User" (any roles)
- they receive an email to activate their account
- ....
- profit (ie: they get a "success" page, and are logged in to the site)

What's actually happening
- add user under "Users", "Add new user"
- submit the form
- user gets a text email, with an Activation link
- Activation page, regardless of the site or theme, gives an error:

"An error occurred during the activation
Invalid activation key."

What else have I tried
- tried disabling ALL plugins
- tried disabling ALL plugins except for Ultimate Member
- tried re-installing Ultimate Member from scratch
- tried re-installing WP from the Updates page

Today, I noticed that adding new users from the main Network admin dashboard seems to work, but this way, I cannot specify the user role, or site access...

I'm too new to understand what's failing, if it's a bug in the system, setting problem, plugin conflict, database error, etc.

Does anyone there have any idea what could be causing this, or where I can look to troubleshoot?

thanks in advanced!

- Ira