AttributeError: 'module' object has no attribute 'PROTOCOL_SSLv3'

After migration seafile server pro from ubuntu 14.04 to ubuntu 16.04 I have an error:

2017-03-20 12:31:56,467 [ERROR] django.request:256 handle_uncaught_exception Internal Server Error: /lib/70ea0490-e07b-4296-b013-021a2f03f144/file/[LEGAL&FINANCE] Rainbow Squad/Agreements/(Mymygames) Videogame Development Agreement dd.16.09.2016 (Game under Working Title Rainbow Squad)-fullysigned.pdf
Traceback (most recent call last):
File “/home/seafile/seafile-pro-server-5.1.5/seahub/thirdpart/Django-1.8.10-py2.7.egg/django/core/handlers/base.py”, line 132, in get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File “/home/seafile/seafile-pro-server-5.1.5/seahub/seahub/auth/decorators.py”, line 26, in _wrapped_view
return view_func(request, *args, **kwargs)
File “/home/seafile/seafile-pro-server-5.1.5/seahub/seahub/base/decorators.py”, line 73, in _decorated
return func(request, *args, **kwargs)
File “/home/seafile/seafile-pro-server-5.1.5/seahub/seahub/views/file.py”, line 342, in view_lib_file
return _file_view(request, repo_id, path)
File “/home/seafile/seafile-pro-server-5.1.5/seahub/seahub/views/file.py”, line 407, in _file_view
from seahub_extra.wopi.utils import get_wopi_dict,
File “/home/seafile/seafile-pro-server-5.1.5/seahub-extra/seahub_extra/wopi/utils.py”, line 6, in
import requests
File “/usr/lib/python2.7/dist-packages/requests/init.py”, line 53, in
from .packages.urllib3.contrib import pyopenssl
File “/home/seafile/seafile-pro-server-5.1.5/pro/python/urllib3-1.7.1-py2.6.egg/urllib3/contrib/pyopenssl.py”, line 42, in
ssl.PROTOCOL_SSLv3: OpenSSL.SSL.SSLv3_METHOD,
AttributeError: ‘module’ object has no attribute ‘PROTOCOL_SSLv3’

Please, give me some advices how I can fix this. Thank you!

Does your document server (from Microsoft? or Collabora) support TLS?

Did’t quite understand what you mean by the ‘document server’? An error occurs when I try to load document preview.