In Ukraine, Russia funds pro-Russia puppet parties to try and undermine the government there. (Ukraine is who they invaded and annexed . Paul Manafort, Trump's second campaign manager, worked for these parties for about a decade. Tad Devine, who did work for Gore/Kerry, worked for these parties and their campaigns for about 4 years before leaving in early 2014 and jumping to the Sanders campaign about 6 months later.
This is a super bad thing to have on your resume, one that makes it hard to believe that you're trustworthy and capable of providing honest advice going forward. (Much like say, Deval Patrick's stint at Bain Capital makes the idea of him jumping into a Dem primary a ridiculous notion.)
Bernie himself isn't implicated at all here other than possible bad judgment in hiring him.