new account fails with webmail, succeeds with Thunderbird

[Michael Maxwell]

New here, so forgive me if I'm posting to the wrong place...

I was creating a new BitBucket account.  Entered my email and pw, it said it was sending me a confirmation email.  I opened the email in my webmail (roundcube, comes from https://roundcube.net), clicked on the confirmation link, and was taken to a login page at bitbucket.  I logged in with my email/ pw, only to be taken back to the page saying they had sent me a confirmation email.  I tried a bunch of things: click on the link, copy-paste the link URL, re-send the confirmation email.  Nothing worked.

Finally I opened Thunderbird, which displayed the email as a formatted html page.  (Roundcube only displayed the html source.)  This worked; instead of taking me to the login page, it logged me in and asked a confirmation question, then let me set up my new repo.

I'm mysterified.  My best guess is that roundcube somehow mungs the confirmation link (which is certainly long, but so what?), but I've never had any problem with it before.  I'm posting here not because I need help (since I finally made it work), but because someone else might have a similar problem, and not be able to make it work.  I copy-pasted the URL from my webmail and from TBird, and there is one difference: my webmail shows an HTML entity '&', whereas in TBird I get just the character '&'.  So perhaps that's it?