Mailbox NonExistent after upgrade 2.3.7 to 3.4.8

Hi all,

I'm trying to upgrade an old server running cyrus imap 2.3.7 to a new one running 3.4.8.
I'm doing a sandboxed test installation of 3.4.8 (the latest available for Almalinux) using a snapshot of my existing data as per the suggestion from https://www.cyrusimap.org/3.4/imap/download/upgrade.html.
Upgrade goes smoother than I anticipated, except for one mailbox in the defaultdomain which keeps being NonExistent.

I've checked and compared all permissions, but I have no idea why this is happening.
This the output of cyradm for two users using the defaultdomain (a working one and the non-working one) and another using a different domain. 
localhost> lm user.sander
user.validuser (\NonExistent \HasChildren)  
localhost> lm user.johan
user.validuser2 (\HasChildren)
localhost> lm [email protected]
[email protected] (\HasNoChildren)  
localhost> lm user.nouser
localhost> 

I've done the upgrade serveral times now and keep getting the same issue. On the old system everything works as expected.
The first start after the upgrade gives some errors, but these seem the same for multiple mailboxes (and looking at the documentation are to be expected):
Apr 29 22:06:57 murphy cyrus/ctl_cyrusdb[4453]: mbentry has no uniqueid, needs reconstruct: mboxname=<!user.sander> syserror=<File exists> func=<mailbox_open_advanced>
Apr 29 22:06:57 murphy cyrus/ctl_cyrusdb[4453]: mbentry had no uniqueid, setting from header: mboxname=<!user.sander> newuniqueid=<4425c29e4edb81d3> syserror=<No such file or directory> func=<fixmbox>

Can anyone point me in the right direction where to look and find out what is happening here?

Thanks, Sander
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