invalid table

19.15 database on Solaris Sparc, a lot of objects are invalid

Including a lot of invalid tables. Names like cpResAssignNotPref974_TAB

Some other invalid objects (views, packages) cannot be compiled because they state:

ORA-04063: table "X.Y" has errors

So I understand that the tables have errors ... but what does that mean ?

I found at least one such a table with an XMLTYPE as column type for such a table, so I presume that be part of the problem.

But it comes back to this: how can this be invalid ? The table was created, and that must have been OK because otherwise the table woumdn't exist anymore. So I guess some alter command was done ... done, but resulting in an invalid object ?!