I get the exact same results you do. With Calibre 0.8.20 and 0.8.21. The mobiperl tools (or more specifically, the Palm/PDB.pm perl library) expects the Palm Database Name to be terminated with a null character otherwise you get the error you're getting:
# database names must include a terminating NUL.
die "bogus database name! is this really a PalmOS file?" unless $name =~ /.+\0/;
So maybe there are certain conditions in calibre conversions that result in a Palm Db Name that isn't null terminated when long titles are involved? Somewhere between 52 and 56 characters?