First thing would be to check the External Data Sources, if you have converted from 5 it may have some old references it is trying to resolve while opening.
go File > Manage > External Data Sources...
Then look at the list and try to tidy these up if there are multiple references to the same file, and if a single ref has old out of date references that may no longer exist.
Be careful not to just delete these as they are most likely used in Table Occurrences and Scripts.
Also save the database as a compacted copy, File > Save File As... this is like running maintenance on a file and sort of tidies it up a little.
Let me know if this helps, and if it is the Data Sources I can explain more on how to tidy this up.