It appears that the file /usr/local/cpanel/3rdparty/mailman/lists/<listname_you_elided>/requests.pck is corrupt.
You can see what's in it with /usr/local/cpanel/3rdparty/mailman/bin/dumpdb.
If you just remove the requests.pck file, I think that will solve the problem, but first use dumpdb to see if there are any real requests.
It appears that the file /usr/local/ cpanel/ 3rdparty/ mailman/ lists/< listname_ you_elided> /requests. pck is corrupt.
You can see what's in it with /usr/local/ cpanel/ 3rdparty/ mailman/ bin/dumpdb.
If you just remove the requests.pck file, I think that will solve the problem, but first use dumpdb to see if there are any real requests.