SharePoint online External Invitation Issue

When external users struggle, we ask them to Clear their browser cache following key steps - close out of everything and Quit Teams - not close, but Quit. Open only one browser page, then clear cache. Restart. 

"When in doubt, clear it out" ;-) 

We also suggest to try InPrivate or InCognito. These have helped many users who have struggled.

One other thing, we typically will provide a separate link to the site or library so they can use that in the future as the initial email is an invitation link.