On the CUSTOMERS page, the LOG IN column is cut off from view (the display has high resolution like 2k+)
Most likely you enabled scaling at Operation System' level for increasing fonts' size on the high-resolution display. Try to decrease the scale or change font size, or you may just scroll down to see the horizontal scrolling button to manage the view.
Resellers see "name_of_your_browser blocked a pop-up from **.cloud.im" screen when they log in as an end-customer. Is that expected?
This behavior is expected. As “log in as” scenario opens a new browser tab, any modern browser requires confirmation of this action.
3. Google Chrome
What Role differences are defined when reseller creates new user with "Admin" or "User" roles?
Admin is a powerful user that can modify an account, basically can do anything they want. User is a service user that can only consume services bought for a reseller own account.
Why can an error notification appear when a reseller user logs in?
Make sure whether the exact user is enabled. Make sure whether there's a user nominated for account admin role.