Cisco switches run PVST by default.  PowerConnect switches do not support PVST and they will forward PVST BPDUs (as well as the other Cisco proprietary protocols) as multicast traffic.  This is probably the cause of your "self loop" errors. The whole network needs to run the same STP protocol or the 2 protocols will determine different spanning-trees.  Since the Ciscos are probably running PVST and the PowerConnects run STP, this is probably the cause of your blocked route. 
I recently tried enabling spanning tree on our 6248 switch stack at our co-location.  We have a direct connection with a route that goes from our HQ to the colo.  When I enabled spanning tree, it completely blocks that route.  I can't even ping the gateway.   I checked the logs on our Juniper firewalls (where the routing is done) and don't see anything indicating an issue in there...it doesn't even seem to be getting that far.  Everything works fine when STP is disabled.  Anyone know what could cause this, or where I might be able to start looking.  Unfortunately I had to restart the switches to get everything working again (to revert back to before I enabled STP) so the logs seem to be wiped out during the time I had it enabled as well. The reason I'm trying to enable STP, by the way, is because the switch stack is also connected to a couple of Cisco routers (owned by the colo).  They have STP enabled and one of the routers is getting a self-loop error since we don't have STP enabled on our switch stack. Any help would be appreciated!
Enabling spanning tree blocks routes to other sites