deposits on DCPR8E spark plugs

What fuel are you running?

Looking at those plugs I'd say you have failed valve stem seals that does that  that buildup is oil ash you most likely not using Rotax oil but a different brand of similar oil.the extra carbon in the cylinders will cause pre-ignition that causes the melting of the carbon look the pre-ignition also fretts the crankcase  is have the engine check out and services repaired by a good rotas service agent. I saw a motor destroyed in 650 hrs as he saved about 500 in servicing costs but that intern cost him a new motor  so he threw away 20k to save 500 

Rotax 9-series engines require thermal paste on spark plug threads to dissipate heat from spark plug into aluminum cylinder head.  Thermal paste should be applied so first two threads remain clean.

Kevin Stewart wrote:

What fuel are you running?

I have always used 100LL with Decalin Runup in every tankfull.

James Stricker wrote:

Rotax 9-series engines require thermal paste on spark plug threads to dissipate heat from spark plug into aluminum cylinder head.  Thermal paste should be applied so first two threads remain clean.

Yes, I have always used the paste and applied it as shown below.  It does appear that there is paste on the first threads but if that is what it is, I have no idea how it got there.