Using Thunderbird 45 and SoGo Connector 31.0.3, as soon as you edit (remove or add) a category on a CardDav contact, that specific contact never gets synced anymore with the server, meaning SoGo won't pull updates from the server and neither will it finish pushing updates. This is the error message from the console:
Upload failure uploading card: https://serveraddress/card.php/addressbooks/User/test/C726ABF9-8FD0-0001-68EC-9BC212DA18DD.vcf.
HTTP Status Code:412

• card properties:
  0 prop: [xpconnect wrapped nsIProperty]; name: DisplayName; value: Diese Kontakt is futsch
  1 prop: [xpconnect wrapped nsIProperty]; name: RecordKey; value: 2
  2 prop: [xpconnect wrapped nsIProperty]; name: PreferMailFormat; value: 0
  3 prop: [xpconnect wrapped nsIProperty]; name: DbRowID; value: 3
  4 prop: [xpconnect wrapped nsIProperty]; name: groupDavVersionPrev; value: "c1b2326e235d3989198de847afc49b6d"
  5 prop: [xpconnect wrapped nsIProperty]; name: PopularityIndex; value: 0
  6 prop: [xpconnect wrapped nsIProperty]; name: PhotoType; value: generic
  7 prop: [xpconnect wrapped nsIProperty]; name: LastModifiedDate; value: 1468939917
  8 prop: [xpconnect wrapped nsIProperty]; name: FirstName; value: Diese Kontakt is futsch
  9 prop: [xpconnect wrapped nsIProperty]; name: groupDavKey; value: C726ABF9-8FD0-0001-68EC-9BC212DA18DD.vcf
  10 prop: [xpconnect wrapped nsIProperty]; name: CardUID; value: C726ABF9-8FD0-0001-68EC-9BC212DA18DD.vcf
  11 prop: [xpconnect wrapped nsIProperty]; name: Categories; value: BRUMM
  12 prop: [xpconnect wrapped nsIProperty]; name: groupDavVersion; value: -1
  13 prop: [xpconnect wrapped nsIProperty]; name: PreferDisplayName; value: 1
• done

The only solution is to remove the contact from the addressbook, remove all it's categories und move it back to the addressbook. This makes working with categories completely impossible. Plus this bug does not trigger any warning to the user so the user won't be aware that his contacts are diverging for weeks!
Editing categories directly via double click on the contact does not trigger this bug, but doesn't allow for mass-editing categories.

Also, this bug makes the use of the extension "Category Manager" (1.62) impossible, because as soon as you activate that extension, all you contacts with categories become unsyncable.