Usually INST errors occur when your database is full. Check by 'dir *'. Remove some datablocks if necessary. When you clean the database and when you look at the space you gain in real/integer or whatever, nothing is updated. until you quit O and restart it.

This is a feature; as soon as you read or sam_at_in something the database statistics will be updated.

Another possibility is that you have an object "without a name" (e.g., consisting of control characters):

  O >  Vis> Error in INST, object  not stored.
 Vis> Error in INST, object  not stored.
 Vis> Error in INST, object  not stored.
 Vis> Error in INST, object _DOT not stored.

Also, you might have deleted the entries in the database for an object. Look at the list of objects on the screen, click on and off each of them to see which is the offending object and then just delete it.