Answer

Here's a few things to check -

When updating the layout table when editing a startcenter template the
contentuid is taken from the contentuid property on the startcenter tag
in the presentation column for that startcenter template (that table is
sctemplate). If that attribute is missing it uses the sctemplateid for
the start center template (typically, they will be the same).

When it writes out the records for layout table, the contentuid for that
tables columns will be the <contentuid> + P + <numberic counter for each
portlet>

So, for example, if the contentuid for the template in question is 42
then the query: select * from layout where contentuid like '42P%' should
show any stranded rows.

Also verify that there are no stranded full sessions. When the
temporary startcenter is created for editing it is added to the scconfig
table with a null groupname, so before removing any layout records
verify that there isn't a pending session being editing by querying the
scconfig table, i.e. select * from scconfig where groupname is null.