Users of BTinternet email accounts have recently seen a variation of these errors:

[IMAP] Cannot open folder ... ([Clientbug] Select additional arguments found after last expected argument)

[IMAP] Cannot open folder ... (unrecognized extra arguments to SELECT command)

We have analyzed the issue in detail, including IMAP logs provided by some of our users and unfortunately, our developers came to the conclusion that it's a server issue which can't be fixed on our side. It seems that the BT updated the feature on their back-end servers but didn't adapt the proxy servers which report the errors. It seems that BT is continuously changing the settings and that's why sometimes the account reset helps but later on the issue reappears.

Some tweaks to the settings seem to help though, these are solutions provided by our users:

"A bit of research reveals that BT uses two versions of mail - BT email and BT Yahoo email. See here
http://bt.custhelp.com/app/answers/detail/a_id/57483/~/im-not-sure-if-ive-got-bt-mail-or-bt-yahoo!-mail

By logging into the desktop version of BT email you can see which version you have got. It transpires I have BT Yahoo mail. I have set the email up using BT Yahoo settings and it is now working again. Something must have changed on BT's side. So if you get this query again this should be the solution."

Another tip that seems to help is filling in the manual option for credentials under the IMAP tab in Menu>Tools>Accounts. Open this section and open the IMAP tab for your BT account. Insert your credentials into "these credentials" fields and save the settings.

Also, if your username is set anywhere as just the user part, make sure to change it to full address - ie. if the full email address is example@btinternet.com make sure the user name is set to "example@btinternet.com", not just "example".