Please verify first if the synchronisation user has sufficient access and rights to/in this company in Exact Online. You can find further explanation about this in this article.
As a Super user, follow the steps below to check this in Basecone:
Go to Settings - Office - tab Accounting system users and click on edit
The Basecone/sync user is mentioned next to User settings
Click on this username underlined in blue
The Basecone user is linked to the Exact user
Does this Exact user have access to this company in Exact?
Can you verify the permission rights of this user in Exact? Does this user have permission to post in Exact?
Does the Basecone application have access to your Exact environment?
You can find this in My Exact Online (via the sync user) at the top right > Security Center > at the bottom of the page click on 'Basecone' underlined in blue > Manage companies. Please put all companies to the right.
If the above Exact user has the right permission access to the company, and the issue persists, the division code might have 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.