SUA stands for Singe User Access. Maybe the company is in Single User Access mode or it needs to be in Single User Access mode.

Thank you for information, on this system I have got below message

"You must call SetDefaultDatabase before trying to use the default connection" when I'm using SDK - have you got maybe idea if it could be connected with previous one ?

Is this running on a webserver or is it a Desktop application?

It is on desktop application

Are you using the COM api or .NET api?

Are you using the COM api or .NET api?

.NET api

Any problems connecting to the Sample companies and importing an Invoice?

How about using the Sample Programs in the .NET Samples Folder? Test out the WinFormExpamples Project, put in your api key and see if you can read and create a new invoice in the Company that you are having problems with.

I think that problem could be that to many times connection to Sage is closed and opened - could it be case ?

Are you referring to the connection from your Custom Application to Sage? Are you creating a connection to Sage for each invoice or are you opening a connection, importing all the invoices then closing the connection? Are you using multi-threading?

I would start with the basics to eliminate possible problems. Using the sample application I would test things step by step.

Using the WinformExample app:
Put in your api key and Click "Begin Session". Does it load the company lists with out any errors?
Try and open the Company that you are working with. (You may have to authorize the app). Does the company Connect with out any errors?

Click the Manage Sales Invoices button. Click the Open Invoice Button. Can you open and view an invoice without any errors?

Click the New Invoice Button. Click the Apply to Sales tab(unless you have a sales order to apply the invoice to).
Select a Customer, put a valid invoice number in, then put in sample info for quantity, item, description, unit price, select a sales tax code and then hit save invoice, does this work with out errors?

If you are still having problems maybe post some of your connection code and the invoice posting code.

Yes, that would definitely do it. If you have closed the company, than any Sage 50 objects you've retrieved in your application would no longer be able to access the data they need.