After a successful recovery of a deleted mailbox, the login button for the restored mailbox is missing from the Plesk interface, and logging in to the restored mailbox via the Roundcube Webmail fails.
If a Plesk mailbox gets deleted, the original password cannot be recovered and used for the restored mailbox.
In one of the upcoming updates to the Plesk integration, the Acronis extension will be prompting to specify a new password for the recovered mailbox. Check the release notes for the fix with ID CI-21600.
As a temporary workaround, please reset the mail account password after the mailbox recovery as per the instruction on the corresponding Plesk help page.