As I recall, you have to have at least one connection to solo mine.
Edit: Also, as I recall, with the version 15.2 (versus the older version), the Bitcoin/Litecoin developers disabled the solo mining (compared to the way it was in the older version of the NDL wallet). It has been a while for me thinking of such things.
Interesting to know this 15.2 feature or lack of feature. MC or Marycoin is 15.6 based coin. I had no problem solo mining MC with GPU or with ASIC.
But same still true, for me to solo mining MC successfully in the past, the wallet syncing had be 100% complete in debug.log file. Without 100% complete syncing, MC solo mining did not work either. NDL wallet I never got 100% synced on wallet. , always 99.99999% complete.