This is a duplicate of #249, please follow up there.
Categories get lost when syncing with ownCloud
Set up sync with ownCloud. Now, on the Android device, add a new field which you do not already have on the contact stored on the server side (note etc) and sync. The new field will be transferred to the server, however the category of the contact will be reset to be empty.
Thanks for your report, but this is auplicate of #48. Please search through the issues before creating a new one.