Control first if the synchronisation user has sufficient access and rights to this company in Exact Online. You can find more explanation about this in this article.
As a Super user, follow the steps below to control this in Basecone:
Go to Settings - Office - tab Accounting system users and click on edit
The Basecone user is mentioned next to User settings
Click on this username in blue
The Basecone user is linked to Exact user xxxxxxx
Does this Exact user xxxxxxx have access to this company in Exact?
Can you control the permission rights of this user in Exact? Does this user have permission to book in Exact?
If the above Exact user has the right permission access to the company, then it is possible that the division code is changed.
As a Super user, you can update this yourself in Basecone. Go to Settings > Company > tab Company connection. Follow the instruction in this link to update the division code.