Okay so basically as of now (Ver 1.02) whenever someone uses dlc cards in his deck it's causing everyone to crash on multisplash however character DLC is okay and won't cause people to crash.
If you don't know which cards are DLC then basically all the cards at the bottom of your cards that have a series of pictures or the 2 maid cards you get are considered DLC cards and cause crashes. Therefore you shouldn't use them and you can ask other players to view your cards in a lobby to see if you're "clean" (If they crash when viewing your profile it means you're using a DLC / bugged card).

In addition to that some people can't find online matches because they are searching in regional search. In order to go into global matchmaking go to Multisplash tab and hover your mouse over the gamemode you want to play and press the V key on your keyboard (Default settings) by pressing V you'll go into global matchmaking and be able to find matches easily.

To sum up:
1) Don't use DLC cards.
2) Check people's profiles before the match starts and try to inform them about this bug or send them here if they cause you to crash.
3) Use global matchmaking in order to find matches more easily.
4) I'll add pictures to this guide and expand it on Friday so everything is clearer! Until then I hope this guide helped you!