Use milder base. Sometimes de-halogenation of the aryl halide is observed. Is your compound having any other base-labile functional groups?

maby your reaction temorature is very high.

you must  decrease the reaction temperature and try tagain.

it must noted that the aryl amine compounds are degradible after formation in the room temperature

For clean N-arylation, you can try Buchwald-Hartwig amination with aryl halide or triflate  with aryl /diaryl or alkyl/dialkyl  amines in presence of Pd-cat, suitable phosphine ligand and base. Plenty of good references are available in literature. Yields are excellent for most of the reactions.

It's not clear from your question what type of substrates you are using for N-arylation. Simple aryl halides are too unreactive for substitution with amines that too in presence of weaker bases like Pot. carbonate. If the aryl halide is activated by electron withdrawing groups (in ortho or para position) then the reaction becomes easier but here the base itself can also act as a nucleophile to react with the aryl halide as side reaction. Use of low temperature can minimize it.

Use of strong base may also lead to N-arylation via benzyne intermediate. But here formation of regio-isomeric byproducts and benzyne dimers are also possible. 

Thank you all for your response.

Please note that I suspect now that my reactant degradation is due to DMSO despite the temperature (110°C).

I used the same reactant for an another reaction in n-BuOH with Hunig's base at 100°C and no degradation products occured.

So, has anyone of you any idea for any  solvent instead DMSO?

In literature I find only DMF and DMA. Are these solvents worth a try or is it possible to have the same effect with DMSO?

You can use both DMF and DMA as  solvent  for N-arylation at  100oC  with K2CO3.