Hi,

I just came across your question. The error message is not really helpful, but here's how to fix it:

• Go to General Settings - Companies
• Make sure you enter a VAT number. 
• If your company/organization is not registered for VAT, enter your national ID (in Belgium, that's your registration number at the KBO). At the end of this procedure, clear the field VAT.

• Go to General settings and go to the Accounting tab
• Complete the settings and register

• Once registered, if you are not a VAT registered company, clear the VAT field on the company page

Good luck!