Hi,
The year end closing voucher is posted on 1 Jan of the following year on this account that you specify in the opening transactions form.

This account must be a balance sheet account.
Maybe instead of the trial balance you should better use a balance sheet report to see what is going on.
If you run your balance sheet as of 30 December let's say, then assets and liabilities won't match because the profit and loss for the current year is the element that makes assets and liabilities balance.
You should thus have a special element in your balance sheet report that takes care of it.
What might help is activating the parameter 'create closing transactions during transfer'.
What this parameter will do is clearing the profit & loss accounts as of 31 December so that your BS matches.
In other words, you get an extra posting made on 31 December and on 1st Jan.
Maybe let your accounting people 'play' around with this parameter setting in a demo/test environment to see if they can understand the transactions better with the parameter activated.
Best regards,
Ludwig