The problem still exists in the X4 version now downloadable from the main website.
Whilst technically thunderbird shouldn’t be attempting ENABLE UTF8=ACCEPT unless UTF8=ACCEPT is declared the IMAP capabilities, the Axigen IMAP implementation in X4 is also at fault; RFC5161 states that:
If the argument is not an extension known to the server, the server
MUST ignore the argument.
If the argument is an extension known to the server, and it is not
specifically permitted to be enabled using ENABLE, the server MUST
ignore the argument. (Note that knowing about an extension doesn’t
necessarily imply supporting that extension.)
By returning an error rather than ignoring ENABLE UTF8=ACCEPT the X4 IMAP service is not compliant with the spec - whereas the X3 service is.
Thank you for the report. This behavior was reported internally and a fix will be developed. Until the fix is available, the above mentioned workaround can be used.