Failed parsing date from field - how to troubleshoot

I'm having this weird problem where Logstash is throwing this error left and right:
{:timestamp=>"2016-10-13T12:08:24.357000+0000", :message=>"Failed parsing date from field", :field=>"log_timestamp", :value=>"%{Date} %{Time}", :exception=>"Invalid format: \"%{Date} %{Time}\"", :config_parsers=>"YYYY-MM-dd HH:mm:ss.SSS", :config_locale=>"default=en_US", :level=>:warn}

I have a date and field coming from separate fields, I combine them and then parse them. In ES I cannot see any errors, everything looks fine in my data.
Here's my config:

grok {
match => ["message", "%{DATA:Date} %{DATA:Time}"]
mutate {
add_field => {
"log_timestamp" => "%{Date} %{Time}"
date {
match => [ "log_timestamp", "YYYY-MM-dd HH:mm:ss.SSS" ]
timezone => "Etc/UTC"

Relevant part of my logfile:

2016-10-13 06:10:50.346 (Disqus truncates the 4 spaces between the two but they're there)

So my question is really: How do I troubleshoot this? In elasticsearch everything looks fine. Im not getting any dateparse errors in my tags. So what's this warning all about?

To follow up on this. I was indeed getting parse failures, but not dateparse errors like I was expecting.

I have no idea why grok was sometimes unable to parse my string, and I could find no identifying factor tying the error cases together. In the end I rewrote the whole thing using the ruby filter plugin instead, splitting everything into an array and then putting every array object into its own field. It's been running flawlessly now for a couple of hours so I'll leave it like this.