The following are a few MTB series:
The Loppet Foundation puts on a bunch of MTB races; mostly out of the Theo Wirth area. https://www.loppet.org/
MN Mountain Bike Series happens non-COVID years: https://www.mnmtbseries.com/
Freewheel Bike normally puts on a Thursday night series: Freewheel Bike Shop - Minneapolis - Twin Cities - St. Paul
There are a couple races up in Cuyuna each year: https://www.cuyunalakesmtb.com/