Not sure if it's the same but recently I too had problems with the update manager on LMDE.
I would suggest you to read this blog post: http://blog.linuxmint.com/?p=1781
about the new update pack system that has just been introduced to LMDE. You should install the package mintupdate-debian (which will remove mintupdate but install the new Debian-optimized one, which will hopefully work.) Then change you sources.list to the incoming or latest repo.
In the new Update Manager there is a button called "Update Pack Info" and if you click on it you can read some useful information, including a fix for Flash issues.
Let us know how it goes, and don't give up so easily
LMDE is currently in a state where a lot of innovation happens and it will be even better than already is.