General:
* Issues
* Troubles
* Difficulties
* Challenges
* Obstacles
* Hardships
* Dilemmas
* Predicaments
* Complications
* Setbacks
More formal:
* Adversities
* Impediments
* Disruptions
* Anomalies
Informal:
* Messes
* Headaches
* Bugs (referring to technical issues)
Specific:
* For a particular type of problem: You can use a more specific synonym like "errors" (for technical problems), "conflicts" (for interpersonal problems), or "disputes" (for legal problems).
* For a problem with a solution: You could use "solutions" or "fixes".
* For a problem that's hard to solve: You could use "impasse" or "deadlock".
The best synonym will depend on the specific situation and the tone you want to convey.