It looks that DBMail uses Cyrus' timsieved implementation. So, it can have the same bug for which there's already a workaround in Net_Sieve package. Please revert your changes and try with this patch (plugins/managesieve/lib/Net/Sieve.php):
--- Sieve.php (wersja 5478)
+++ Sieve.php (kopia robocza)
@@ -1186,7 +1186,7 @@
// The server should be sending a CAPABILITY response after
// negotiating TLS. Read it, and ignore if it doesn't.
// Doesn't work with older timsieved versions
- $regexp = '/^CYRUS TIMSIEVED V([0-9.]+)/';
+ $regexp = '/TIMSIEVED V*([0-9.]+)/i';
if (!preg_match($regexp, $this->_capability['implementation'], $matches)
|| version_compare($matches[1], '2.3.10', '>=')
) {