After upgrading from 7.3.0 to 8.0.0, Why does Splunk Web server turns out the ERROR message?

[yeaobin]

I upgraded splunk enterprise instance from 7.3.0 to 8.0.0.

All preliminary checks passed when splunk restarting.
But when i accessed to the web interface,it turned out this python error:

500 Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

Traceback (most recent call last):
File "/app/splunk/lib/python3.7/site-packages/cherrypy/cprequest.py", line 628, in respond
self._do_respond(path_info)
File "/app/splunk/lib/python3.7/site-packages/cherrypy/_cprequest.py", line 659, in _do_respond
self.get_resource(path_info)
File "/app/splunk/lib/python3.7/site-packages/cherrypy/_cprequest.py", line 749, in get_resource
dispatch(path)
File "/app/splunk/lib/python3.7/site-packages/splunk/appserver/mrsparkle/lib/i18n.py", line 405, in __call_
request.lang, request.mofile, request.t = get_translation('messages', [locale])
File "/app/splunk/lib/python3.7/site-packages/splunk/appserver/mrsparkle/lib/i18n.py", line 550, in get_translation
t = translations.setdefault(key, SparkleTranslations(open(mofile, 'rb'))) # was GNUTranslations
File "/app/splunk/lib/python3.7/gettext.py", line 259, in __init_
self._parse(fp)
File "/app/splunk/lib/python3.7/gettext.py", line 420, in _parse
catalog[str(msg, charset)] = str(tmsg, charset)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe7 in position 0: ordinal not in range(128)

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[cjpote]

I've got the same problem; did you ever figure it out?

[srauhala_splunk]

I had a situation where Splunk web refused to start after upgrading to SE 8.0.1, with UnicodeDecodeError. In our situation if was due to the system locale being set to sv_SE. Changing it to en_GB helped.

[nickhills]

Did you run through the migration checks before upgrading?
https://docs.splunk.com/Documentation/Splunk/8.0.0/Installation/AboutupgradingREADTHISFIRST

Take a look at this post following and see if it addresses your issue.
https://answers.splunk.com/answers/779171/upgrading-from-701-to-80-splunk-web-stopped-runnin.html

[nickhills]

I think I have seen another post on the subject - and after a quick look now, I can't find it!

There was a suggestion that Splunk 8 does not handle dashboards that do not have a label, and can trigger this message.
Sadly the message does not give any indication as to which dashboard is the problem.

The suggested advice was to look through the xml for your dashboards and identify any which do not have a label and manually edit the xml to include one.

If I can find the post I will update my answer to give credit/context.

EDIT### 

I have retracted my answer, as I was mistaken. Here is the post I was referring to, but it does not apply to your situation.
https://answers.splunk.com/answers/802002/500-internal-server-error-when-i-click-manage-apps.html