Sometimes, error messages may appear. What to do?
1 - READ WHAT THEY CONTAIN: pressing the Esc key or clicking on the red cross will certainly not solve the problem.
2 - If the messages are explicit, like :

Follow what they say. (In the above case, wait until the other user has exited).
3 - If the messages find an anomaly, such as (when exiting the calculation) :

Find the source of the problem and make the necessary corrections (in the example above, redo the 13th accumulations)
4 - If the message mentions an index problem,

Go to "Diagnostic" - "Tools" - "Tools for tables" - "Check and rebuild data" :

Choose the table mentioned in the error message and click on "Rebuild" then "Verify" :

This should be enough to solve the problem.
If the company cannot be entered at all, the tool is available from the company list: :

5 - The error message contains a problem with locks in a company
Something like problem + company name followed by PDOXUSRS.LCK
You should physically remove (delete key) the files with the extension .LCK from the disk:

6 - If the messages note problems that you do not know how to solve, such as :

MAKE A SCREEN COPY OF THE MESSAGE AND PASTE IT IN AN EMAIL TO: sav@git.ch telling us what part of the program you were in so we can help you out.