BAD : I experienced problems to find the right accommodation.
GOOD : I experienced problems in finding the right accommodation.
have/experience a problem/problems (in) doing sth (NOT to do ): 'You shouldn't have any problems in getting a visa.'
BAD : Unemployment is a very important problem.
GOOD : Unemployment is a very serious problem.
a serious problem (NOT important ): 'Teenage vandalism is a serious problem in this area.'
BAD : When population growth is not controlled, serious problems can happen.
GOOD : When population growth is not controlled, serious problems can arise.
problem + arise / occur (NOT happen ): 'How did the problem first arise?'