I cannot see the graphics. Tried with different browser. any idea of why this disappeared just creating invoice and customers?
Hi,
The grey screen usually means there were an error rendering the page, either from a bug in the code or from bad data.
If you can please try with the v5 mobile app you may see a specific error message.
Related to this, were you able to use the migration tool or did you manually migrate the data?
All done manually 
The Overview was working, something done creating customer (3) and invoice made it all grey.
Where to find the mobile app? OK Found.
If I access from the mobile app is all grey after the login. No error, but the app is impossible to use.
Ok, we plan to release a new version of the mobile app in a few days which will have more logging enabled.