Hi,
Some people had Warnings and errors when running brian2 on windows (after following the miniconda Tutorial). One solution seems to be the following:
First download and install Microsoft Visual C++ Compiler for Python 2.7
https://www.microsoft.com/en-us/download/details.aspx?id=44266
Once it is done go back to your (bmnn) environment on conda prompt and run:
python -m pip install --upgrade ipykernel==4.9.0
UPDATE (Thanks Santoro Matteo) : it seems also necessary to run:
conda update matplotlib
Please Let us know if it has solve your issue. (if not please post the complete error message)