Marker stone formspecs fail on 0.5.0-dev

[nyje]

After moving 3 different servers from 0.4.16 (where all was ok) to 0.5.0-dev, I have only now found on playtest that the formspecs of the marker stone fail, with the following displayed on the server's console:

'nyje' submitted formspec ('markers:info') but server hasn't sent formspec to client, possible exploitation attempt

and obviously no actions are taken. I understand the formspec api may have changed a bit?

I looked at the code, and on first glance it will take quite a while for me to root out the problem, are you perhaps interested in making the code compatible with 0.5.0?
(Or pointing out in what way I'm mistaken?)

markers: commit ad3c999
minetest 0.5.0-dev: commit 15bafa60 (Tue Mar 20 02:30:25 2018 +0000)