The “standard” NodeID’s have changed over the years, so it is possible that your emonTx and your emonhub.conf have got out of step.
Your solution is to simply copy the setting for an emonTx - ideally one that is working - and change the NodeID to “15”:
[[15]]
nodename = emontx3
[[[rx]]]
names = power1, power2, power3, power4, vrms, temp1, temp2, temp3, temp4, temp5, temp6, pulse
datacodes = h,h,h,h,h,h,h,h,h,h,h,L
scales = 1,1,1,1,0.01,0.1,0.1, 0.1,0.1,0.1,0.1,1
units =W,W,W,W,V,C,C,C,C,C,C,p