This may be timing out due to the number of lines of the Sales Invoice it is trying to post or could be related to other 'Environmental' factors such as slow or bad performing WebServers/SQL Servers.
To resolve this issue Support will need to check configuration settings and run some scripts on the database. Therefore please raise a new case online and reference the title of this article and please include the Sales Invoice Number which you are trying to approve/post for which this Error is occurring.