"Downgrading is possible to v2.2.27 and later. (v2.2.27 accidentally broke
dovecot.index* backwards compatibility a bit.) "
I've thrown caution to the winds, and downgraded back to 2.2.22, by turning off
the Dovecot-PPA, deinstalling 126.96.36.199, and installing 2.2.22 again from the
Ubuntu package sources. Then I could at least log in to the dovecot server
But i still needed Pigeonhole 0.4.14, and in 2.2.22 there is only 0.4.13. "IMAP
Sieve" is not added before 0.4.14. I have a Kubuntu 16.04.3 LTS System, and the
contained version is only 2.2.22. So I downloaded and installed the needed
dovecot packages from the newer Ubuntu version 17.10. That's version 2.2.27.
And now - I can log in, and there are no complaints about the
IMAPSieve-configuration not being recognized. (I need IMAPSieve for the
training of a spam filter). Now I can continue with that spam filter.
So far, I habe no problems with the broken indexes - yet(??) (see above).
Eventually it can be said that the problem is probably an incompatibility
between the dovecot wich ich packaged for Ubuntu, and the version from the