I have some tables. While validation works for one with a similar
construct, the other table appears faulty

This works:

<simplesect>
            <title/>
            <table>
                <title>Rote Liste</title>
                <tgroup cols="4" align="center">
                    ...

and this does not work:

 <para>Mit den eigentlichen Bläulingen (Bl) wird die Sache schwieriger.
                <table frame="void">
                    <title>Bläuling noch nicht fotografiert</title>
                    <?dbfo keep-together="always" ?>
                    <tgroup cols="4" align="left">
                        <colspec colname="c1" colnum="1"
colwidth="109.2pt"/>
                        ...

can you please explain. I have this error regularily that a table should
have a caption and not a title.

Cheers Bernhard