RuntimeError: b'no arguments in initialization list'

to make sure this is pyproj error rather than geopandas.

import pyproj
pyproj.Proj("+init=epsg:4326")

if the above runtime error is the same, we can be sure this error is due to pyproj.

just conda remove pyproj and install it with pip.

pip install pyproj

at least this works for me.

Today(July 30), I resintalled from miniconda, conda remove pyproj did not work for me, instead I pip uninstall pyproj and pip install pyproj makes everything fine.

The problem is problably within the pyproj instalation of Anaconda on Windows platform. Just like Stephen said, solution is to edit the path in "datadir.py" (located in ...Anaconda3\Lib\site-packages\pyproj).

Correct path is ".../Anaconda3/Library/share". Make sure full path is complete (may contain username etc.). I also needed to change \ to /. This change worked for me. Yes and after this change, it is necesary to restart Spyder (or whatever you use).

Is there an initial crs defined? I ran into the same problem only when I passed only the epsg command: gdf.to_crs('epsg:4326').

As you show

my_geoseries.crs = {'init' :'epsg:3857'}

should be the first step and then transforming to

gdf = gdf.to_crs({'init': 'epsg:4326'})

If you are working in ArcGIS you could also check in the properties whether the initial epsg is defined ?