Finally.
Upgraded to 3.2.6 yesterday evening.
After manual conversion to "improved_mboxlist_sort: 1" everything works as expected, both in "subscribe" and "no subscribe" client modes. Many thanks to Yadd for assistance!
The bug itself seems to be even more smart. In my cluster it appeared only in the oldest backend running since 2005 with multiple upgrades on-place. Another, newer backend started at Cyrus version 2.4 has approximately same structure complexity including non-ASCII folder names but did not indicate any signs of the bug.
Alexey
