ctl_conversationsdb causes a segmentation fault

Hello,

I got an old mailsystem to upgrade running cyrus 2.5.10 on a debian
stretch. Upgrading this system to debian buster, running cyrus 3.0.8 as
a first step, following the upgrade advisories I ran into the following
issue:

ctl_conversationsdb ran for about a day I guess and finally ended with
an segmentation fault.
Investigating the issue i figured out, that it is caused by one specific
user mailbox about 43 Gb in size. Other similar Mailboxes produce a
conversations db of under 70 Mb, but for this particular one, the db
grows and grows and grows until it's 4 Gb in size and
ctl_conversationsdb crashes.

Any ideas what could cause this issue and how to debug this?

My guess would be some mail that causes ctl_conversationsdb to get
somehow stuck in a loop, if that's possible.

Best regards
Christian

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