Hi Jaap, hi Daniel,
There has been no release of MercurialEclipse after Eclipse 2012-12. Therefore, I suspect that this issue is caused by a change in the Eclipse API: so far, the plugin was using Neon.R (4.6.0) as its baseline API. There was probably an API change in 2012-12 (version 4.22.0), which would mean that it's time for the baseline API to move :-)
I'll look into this issue starting Monday next week. If you can create a ticket on Heptapod to track the issue, that'll help. Otherwise, I'll do it when I start working on a fix.
In the meantime, I recommend downgrading to Eclipse 2012-09 if possible.
Thank you for reporting this issue.