Changed password, can't access email

[#1]

I'm a former Plusnet broadband customer who still uses Plusnet email with the free domain Plusnet provided many years ago (long story). I needed help with renewing my domain today, so before calling the call-centre, I accessed my account which necessitated changing my password as I forgot the old one. That went fine (got the email link, changed the password then logged in successfully). I also passed the call centre security check using letters from the new password

However, when I tried to check email later, I was asked for my new password but it didn't work. Tried webmail and it didn't work there either. I then tried to log into my account and it no longer works there! I know the new password is correct as it saved in my Password Manager. Not sure what has happened or how to proceed. Worried that this might get complicated as I'm not a current paying customer. Any ideas?

[#2]

Email applications can mask issues, presenting all failures as being a password problem.  Try the email connection diagnostics using the link below.

[#3]

Unfortunately as I said, my reset password no longer allows me to log in to try this.

[#4]

@Uncle_Tupelo   The diagnostic tests are testing connectivity issues to the email server, you do not need to be logged into anything. They are run directly from your PC using the command line interface.  

https://community.plus.net/t5/Email/Diagnosing-email-server-connectivity-issues-the-guide/m-p/200795... 

[#5 · Accepted solution]

Thanks to those who responded with suggestions. The solution was slightly weird.

Support at Plusnet changed my password to a temporary one which worked on Thunderbird, Webmail and to log into the Plus website. However, when I changed that password to a permanent one which was a variation on a theme (and only a character away from the one I changed to yesterday), it worked on Thunderbird, but not on webmail, the Plusnet website nor on my mail app on my phone.

I then changed the password to something totally different with no similarity to the previous one and that works perfectly everywhere. Odd, but it's fixed.