Opening any notebook gives "500 internal server error" in Raspberry Pi 4. How to resolve it?

Question

I installed Jupyter Notebook on Pi 4 intially it worked properly but now its giving 500: INTERNAL SERVER ERROR. So, I tried to install with conda and, i install Miniconda. when I try to install Notebook with conda it gives fillowing error:

pi@raspberrypi:~ $ conda install notebook
Fetching package metadata: ......
Solving package specifications: ..
Error: Unsatisfiable package specifications.
Generating hint: 
[      COMPLETE      ]|###################################################| 100%


Hint: the following packages conflict with each other:
  - notebook
  - python 3.4*

Use 'conda info notebook' etc. to see the dependencies for each package.

so i tried conda info notebook it gives following:

pi@raspberrypi:~ $ conda info notebook
Fetching package metadata: ......

notebook 4.2.3 py36_0
---------------------
file name   : notebook-4.2.3-py36_0.tar.bz2
name        : notebook
version     : 4.2.3
build number: 0
build string: py36_0
channel     : rpi
size        : 5.5 MB
arch        : armv7l
binstar     : {'channel': 'main', 'owner_id': '5848a1daf838f90fe7b7e9c2', 'package_id': '5852258e2ab33e0fe796edc5'}
has_prefix  : True
license     : BSD 3-clause
machine     : armv7l
md5         : ee4a4bc7e1b2ef397350bafc186dfcfe
operatingsystem: linux
platform    : linux
subdir      : linux-armv7l
target-triplet: armv7l-any-linux
installed environments:
An unexpected error has occurred, please consider sending the
following traceback to the conda GitHub issue tracker at:

    https://github.com/conda/conda/issues

Include the output of the command 'conda info' in your report.


Traceback (most recent call last):
  File "/home/pi/miniconda3/bin/conda", line 5, in <module>
    sys.exit(main())
  File "/home/pi/miniconda3/lib/python3.4/site-packages/conda/cli/main.py", line 194, in main
    args_func(args, p)
  File "/home/pi/miniconda3/lib/python3.4/site-packages/conda/cli/main.py", line 201, in args_func
    args.func(args, p)
  File "/home/pi/miniconda3/lib/python3.4/site-packages/conda/cli/main_info.py", line 180, in execute
    pretty_package(pkg)
  File "/home/pi/miniconda3/lib/python3.4/site-packages/conda/cli/main_info.py", line 142, in pretty_package
    for env in app_is_installed(pkg.fn):
  File "/home/pi/miniconda3/lib/python3.4/site-packages/conda/api.py", line 107, in app_is_installed
    for fn2 in os.listdir(envs_dir):
FileNotFoundError: [Errno 2] No such file or directory: '/home/pi/envs'