According to http://www.swox.com/gmp/#TRY it's a good factor
Just paste
((28433*2^10290193)+1) mod 15481222456892501
in the demo 1 box.
The result is 0, thus a factor.
Does this mean there's a problem with the factor submission form. Didn't something like this happen a year or two ago following a s/w upate (or something) on the server.