Hello Claus,
This error message is displayed when the record's number is not matching the definition set up in Number Series setting.
There can be several cases when this occurs:
1. You have entered some number manually to the field No. on a record - in this case, before saving just select the "Magnifying glass" icon on the field No. and the system should give you a list of available number(s)
2. You do not have your Number Series setup for the year 2018.
In Standard Accounts, it's an InApp, which you have to subscribe to, in order to set the Number Series for the next year(s).
If you have it subscribed, all you need to do is check the setting Number Series for the relevant record you are trying to save. For example, if this is Nominal Ledger Transaction, you need to check the setting in NL module >> Settings >> Number Series - Transactions. Make sure you have a row there that would have 2018.01.01 as your From date.
Let me know if that worked out. Thank you!
Best regards,
HansaWorld Support