Hello,
there is some issue with crating sub folders from iPhone.
I am not sure if it is from the iPhone side or sogo though.
We are using INBOX. for prefix in imap.

This is how the requests looks like:

S[0x5Mar 30 15:53:38 sogod [25473]: |SOGo| starting method 'POST' on uri '/SOGo/Microsoft-Server-ActiveSync?User=user@domain.com&DeviceId=...&DeviceType=iPhone&Cmd=FolderCreate'
Mar 30 15:53:38 sogod [25473]: <0x0x5571deb41660[SOGoActiveSyncDispatcher]> EAS - request for device ...: <?xml version="1.0"?>
<!DOCTYPE ActiveSync PUBLIC "-//MICROSOFT//DTD ActiveSync//EN" "http://www.microsoft.com/">
<FolderCreate xmlns="FolderHierarchy:">
 <SyncKey>MailApp4_6373_27f49843_0</SyncKey>
 <ParentId>0</ParentId>
 <DisplayName>Testbox3</DisplayName>
 <Type>12</Type>
</FolderCreate>

ParentId is 0 for some reason.

This is how the sogo request towards the IMAP server looks like:
[0x5571decd4e00]: * ID ("name" "Dovecot")
S[0x5571decd4e00]: 3 OK ID completed (0.001 + 0.043 + 0.043 secs).
C[0x5571deaebc40]: 4 namespace
S[0x5571decd4e00]: * NAMESPACE (("INBOX." ".")) NIL NIL
S[0x5571decd4e00]: 4 OK Namespace completed (0.001 + 0.043 + 0.042 secs).
C[0x5571deaebc40]: 5 LIST "" ""
S[0x5571decd4e00]: * LIST (\Noselect) "." ""
S[0x5571decd4e00]: 5 OK List completed (0.001 + 0.044 + 0.043 secs).
C[0x5571deaebc40]: 6 create "Testbox3"
S[0x5571decd4e00]: 6 NO Client tried to access nonexistent namespace. (Mailbox name should probably be prefixed with: INBOX.) (0.001 + 0.000 secs).
C[0x5571deaebc40]: 7 logout

This is how working request looks like (Outlooks as client):

Mar 30 16:04:06 sogod [25481]: |SOGo| starting method 'POST' on uri '/SOGo/Microsoft-Server-ActiveSync?Cmd=FolderCreate&User=user@domain.com&DeviceId=...&DeviceType=WindowsOutlook15'
Mar 30 16:04:06 sogod [25481]: <0x0x5571deac2e20[SOGoActiveSyncDispatcher]> EAS - request for device ...: <?xml version="1.0"?>
<!DOCTYPE ActiveSync PUBLIC "-//MICROSOFT//DTD ActiveSync//EN" "http://www.microsoft.com/">
<FolderCreate xmlns="FolderHierarchy:">
 <SyncKey>MailApp4_6389_27f498da_2</SyncKey>
 <ParentId>mail%2F5ee836084f6b29594c2b00008cb2e447</ParentId>
 <DisplayName>TestFolderKo1</DisplayName>
 <Type>12</Type>
</FolderCreate>

and IMAP part:

C[0x5571dec60940]: 9 create "INBOX.TestFolderKo1"
S[0x5571def46670]: 9 OK Create completed (0.013 + 0.000 + 0.012 secs).
C[0x5571dec60940]: 10 subscribe "INBOX.TestFolderKo1"
S[0x5571def46670]: 10 OK Subscribe completed (0.005 + 0.000 + 0.004 secs).