You might try reading the log. It says many things.
Final error:
2020-01-23 10:10:13,763 - tornado.application - ERROR - Uncaught exception GET /downloads/logs/octoprint.log (::ffff:192.168.1.148)
Traceback (most recent call last):
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/web.py", line 1512, in _execute
result = yield result
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/gen.py", line 1069, in run
yielded = self.gen.send(value)
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/web.py", line 2422, in get
yield self.flush()
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/web.py", line 953, in flush
return self.request.connection.write(chunk, callback=callback)
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/http1connection.py", line 437, in write
self._pending_write = self.stream.write(self._format_chunk(chunk))
File "/home/pi/oprint/local/lib/python2.7/site-packages/tornado/http1connection.py", line 412, in _format_chunk
"Tried to write more data than Content-Length")
HTTPOutputError: Tried to write more data than Content-Length
2020-01-23 10:10:13,765 - tornado.general - ERROR - Cannot send error response after headers written
This could have been attempting to download the octoprint.log perhaps. You have several plugins which could be causing the trouble. Try Safe Mode to compare.
Problem with bettergrblsupport:
2020-01-23 09:48:36,476 - octoprint.plugin - ERROR - Error while calling plugin bettergrblsupport
Traceback (most recent call last):
File "/home/pi/oprint/local/lib/python2.7/site-packages/octoprint/plugin/__init__.py", line 219, in call_plugin
result = getattr(plugin, method)(*args, **kwargs)
File "/home/pi/oprint/local/lib/python2.7/site-packages/octoprint_bettergrblsupport/__init__.py", line 241, in on_after_startup
self.deSerializeGrblSettings()
File "/home/pi/oprint/local/lib/python2.7/site-packages/octoprint_bettergrblsupport/__init__.py", line 282, in deSerializeGrblSettings
self.grblSettings.update({int(set[0]): [set[1], self.grblSettingsNames.get(int(set[0]))]})
IndexError: list index out of range
You've disabled the Printer Safety Check.
Looks like one of the plugins is causing this:
2020-01-23 09:48:20,817 - py.warnings - WARNING - /home/pi/oprint/local/lib/python2.7/site-packages/octoprint/util/__init__.py:658: UnicodeWarning: Unicode unequal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
if value_source != value_target:
You have many, many plugins installed. If you're having difficulties printing then it's time to disable all of the add-on plugins and only add them back in when you know they're not causing you problems.