Yep.

https://tools.wmflabs.org/interaction-timeline/?wiki=testwiki&user=198.73.209.241&user=TBolliger%20(WMF)&startDate=1520640000

It's because that IP address has no edits on testwiki... You can do the same thing with a known username:
https://tools.wmflabs.org/interaction-timeline/?wiki=testwiki&user=TBolliger%20(WMF)&user=Josh&startDate=1520640000

I tested with https://test.wikipedia.org/w/index.php?title=Candy&action=history

and a previous IP edit also shows the same error: https://tools.wmflabs.org/interaction-timeline/?wiki=testwiki&user=TBolliger%20(WMF)&user=198.73.209.3&startDate=1520640000

In T191587#4117555, @TBolliger wrote:

I tested with https://test.wikipedia.org/w/index.php?title=Candy&action=history

and a previous IP edit also shows the same error: https://tools.wmflabs.org/interaction-timeline/?wiki=testwiki&user=TBolliger%20(WMF)&user=198.73.209.3&startDate=1520640000

ok, legit. :P

https://test.wikipedia.org/wiki/Special:Contributions/198.73.209.3

Yeah, the issue here is that we try to load the user id from the user name in the request. It obviously doesn't work for IP(s)

It should either be fixed to work with IP's, or fix the workflow to result in a graceful denial instead of a protocol error. I'd much prefer the former.

We want to support querying for IP address users. This is a regression from our work in T180653: Interaction Timeline V1: Support IP users

In T191587#4118420, @Xaosflux wrote:

It should either be fixed to work with IP's, or fix the workflow to result in a graceful denial instead of a protocol error. I'd much prefer the former.

The bug is valid. This was an oversight on my end when working on performance improvements. It will work for IP(s).

https://github.com/wikimedia/InteractionTimeline/pull/86