Yesterday I had the issue that I had to enable a mailfile for IMAP access. This normally
is done with

load convert -e mail/xxx.nsf.

But when trying to enable IMAP I received the following error:

[11AC:0002-4218] 28.09.2021 12:13:55 Mail Conversion Utility starting
[11AC:0002-4218] 28.09.2021 12:13:55 Mail Convert: Started enabling NSF support for IMAP in ‘mail\comptahab.nsf’
[11AC:0002-4218] 28.09.2021 12:13:55 Mail Convert Failed on mail\xxx.nsf: Error enabling NSF support for IMAP: Cannot migrate R5 IMAP Seen flags to Unread marks.
[11AC:0002-4218] 28.09.2021 12:13:55 Mail Conversion Utility shutdown

From the HCL Support I received the information that some issues can cause this error:
Some mailfiles lost these flags on conversion, particularly those that were never accessed by Notes clients or having missing owner info or corrupted calendar profile. Due to this while converting the mailfile you are receiving the error.

How to resolve this issue:

Check the owner info is correctly updated in mail file preference.

Check if the calendar profile is missing using notespeak or if any corruption please recreate the same by using all in one admin tool or Lotus script. Please refer to below links.
https://support.hcltechsw.com/csm?id=kb_article&sysparm_article=KB0029803
https://support.hcltechsw.com/csm?id=kb_article&sysparm_article=KB0037174

For me both links did not work and I added the following entry in the NOTES.INI:

“IMAP_CONVERT_ALLOW_LOST_SEEN_FLAGS=1”
This will revert behavior and allow conversion to succeed with loss of /Seen flags.

After adding this entry and restarting the IMAP server the conversion ran fine.