Incorrect datetime format on hovertool with python bokeh

I've got a issue with the datetime format on my hovertool. I'm not sure but i think since i've updated bokeh and pandas to the latest version (bokeh: v2.0.1 / pandas v1.0.3) in conda the presentation of the dates in my bokeh plot are faulty.

import pandas as pd

# first of all i create a columndatasource and convert my datetime-strings to pandas datetime-objects:

Source = ColumnDataSource(data={'date' : pd.to_datetime(df_data['Date'], format='%Y-%m'), 
                                'values' : df_data['values']})

#print(pd.to_datetime(df_data['Datum'], format='%Y-%m'))
# >>>
#0 1980-01-01
#1 1980-02-01
#2 1980-03-01
#...

hover = HoverTool(tooltips = [('Month', '@date{%b, %Y}'),('Value', '@values{0.0} °C')],
formatters = {'date' : 'datetime'}, mode='vline')

plot = figure(x_axis_label='time',
              x_axis_type='datetime',
              y_axis_label='diff',
              tools = [hover, 'pan', 'box_zoom', 'wheel_zoom', 'reset'],
              background_fill_color = '#2F4F4F',
              output_backend="webgl",
              toolbar_location='right',
              title="Title")


plot.vbar(x = 'Month',top = 'Value', source=Source, width=timedelta(days=20), color='color')

But if i run this, it shows me for "Month" in my hovertool something like "80TiB %" and not "Apr 2020".