==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.9
- Deployed with iRedMail Easy or the downloadable installer? No
- Linux/BSD distribution name and version: Ubuntu 18.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): SQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Im trying to move the emails from old server to new server. 
1) I created a same account (abc@domain.com) which was on old server on the new server.
2) Copied Maildir folder from old server to new server
var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46/Maildir

When I open the email via Roundcube I get an error

Server Error: UID THREAD: Internal error occurred. Refer to server log for more information.

When I check the logs on the server I get the below error.  It seems like the server is not able to read the data I copied.

*******************************************************************************
55:11 mail dovecot[28262]: doveconf: Warning: SSLv2 not supported by OpenSSL. Please consider removing it from ssl_protocols.
Aug 20 18:55:11 mail dovecot[28262]: config: Warning: SSLv2 not supported by OpenSSL. Please consider removing it from ssl_protocols.
Aug 20 18:55:15 mail dovecot[28262]: imap-login: Login: user=<abc@domain.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=28278, secured, session=<GA89/5CQ8Id/AAAB>
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Error: open(/var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir/dovecot.index) failed: P
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Warning: Maildir /var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir: UIDVALIDITY change
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Error: open(/var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir/dovecot.index) failed: P
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Warning: Maildir /var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir: UIDVALIDITY change
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Error: open(/var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir/.Projects/dovecot.index
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Error: open(/var/vmail/vmail1/domain.com/a/b/c/abc-2019.08.20.18.30.46//Maildir/.Projects/cur/151277823
Aug 20 18:55:15 mail dovecot[28262]: imap(abc@domain.com): Logged out in=759 out=4296
****************************************************************

When you create an email account, the user directory where their email is located is time stamped as pat of the directory name in the database. It is covered in the Docs.

You have to then visit the vmail database, then edit the mailbox table -> storagebasediretcory to what you have in /var/vmail/vmail1/<domain name>/<user name>.

The time stamp in the database will be newer than those old folder, thus will not match. SO you have to edit what the old folders have in the new database.

The folders have a default format of /var/vmail/vmail1/<domain name>/<first character user name>/<second character user name>/third character user name>/<user name-date-time>

You also need to make sure the ownership for those folders are for vmail