I met an matplotlib backend error just after Spyder launched
(Spyder maintainer here) This is a bug in Spyder because we still offer the possibility to set the Qt4 backend, but Matplotlib removed it some versions ago. So please change your backend to other value.
We'll fix this problem in our 5.3.0 version, to be released in March 2022, by removing that option.
Today when I launched my Spyder the IPython console immediately showed me a note:

raise ValueError(f"Key {key}: {ve}") from None
ValueError: Key backend: 'qt4agg' is not a valid value for backend; supported values are ['GTK3Agg', 'GTK3Cairo', 'GTK4Agg', 'GTK4Cairo', 'MacOSX', 'nbAgg', 'QtAgg', 'QtCairo', 'Qt5Agg', 'Qt5Cairo', 'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']

I tried to update matplotlib in anaconda command line, but after the updating it still appeared. How can I cope with this problem?