My trial balance is out of balance, but I don't have any fund balance error messages when I run that report. How do I find the problem?

A trial balance run for the same period as a fund balance report should show the same out of balance amount that the fund balance report shows.
If you run the trial balance for period 0 for the same year you typically will get the same out of balance amount.  This will prove that the problem did not occur in this year.  
Rerun the trial balance and back up a year to the prior year and period 12 or 13.  If the same variance exists, confirm that period 0 for this year is also out of balance.  Back up another year and repeat until you find a beginning balance that IS in balance and confirm that the ending year balance is in balance, but the beginning of the following year is not.
99% of the time the problem is caused by a revenue or expense that had no fund balance account to roll into. Please review Why am I getting a missing fund balance for account error on the balance tab in Funds? for information on how to resolve the issues you find.

Since you are out of balance, either the GL account causing the problem will need to have its account type modified or the fund will need a fund balance added.  Once this is handled, run a fund balance report for the end of the year where you found the problem.  Once the fund balance report for the end of the year is clean, run File maintenance > General Ledger > End of year closing for this year ( do not check off any boxes) and rerun either the fund balance report or a trial balance for the more recent year and period 0.  This year should now be in balance.  Run EOY closing for each subsequent year up to the present.  Check either period 0's trial balance or fund balance report as you move towards the present to be sure the changes are moving with you.