Index pattern empty when choosing @timestamp

Hi there

I am busy moving from an on premises Kibana 6.3 to an Elastic Cloud 7.4 instance. So far all is good, except that when I choose @timestamp when creating an index pattern, no data shows in the Discover page in Kibana. When I select no timestamp field, I can see all the data so I know for a fact all the data is in the index (the document count also increases as I write entries to my log files being ingested, so all good).

Here is the date format in my log file and the "target" field section of my ingest file:

Log File data format: 2019-09-26 06:49:41,028

Grok Pattern:
%{DATESTAMP:python_logs.entries.logtime} %{DATA:python_logs.entries.level} WHERE: %{DATA:python_logs.entries.where}\tMESSAGE: %{GREEDYDATA:python_logs.entries.errormessage}

Ingest Pattern:
"date": {
"field": "python_logs.entries.logtime",
"target_field": "@timestamp",
"formats": ["YYYY-MM-DD HH:mm:ss,SSS"]
}

Any help will be greatly appreciated.

Regards

Upon further investigation, I found this:
(I delete and let Filebeat recreate the index with every test I do to avoid other issues)
When I create an index pattern and dont specify a timefield, then I can see all my data and I have this error:
error.message Text '2019-10-07 12:27:18,519' could not be parsed at index 2

When I create an index pattern and specify @timestamp as the time field, then I don't see any data.

Judging by that error message the problem is with the date parser. The Logstash or Filebeat forums will be the best place to get help for that. Over here there's less people familiar with Grok patterns.
You could also try using the Grok debugger from the Kibana Dev Tools in toder to pinpoint the problems