You can try using this RPC call (paste it into the debug console)
What it does is determine the balance based on
742804 confirmations, which means any coins that have moved since then won't be included in the balance.
Also, another thing to try is to start a reindex of the wallet, in the wallet repair menu item, this will rescan the block chain, your wallet balance should show as zero. watch it closely and when the data (see the information screen) gets to December 2020, pull the network plug, or go to airplane mode, or turn off the router, etc to kill the internet and stop the syncing, the balance it shows then, will be correct as of that block. Actually, since that is just a reindex, it might not stop syncing, in which case, be prepared to snip the balance of the wallet, with the snipping tool, eg SHIFT+WIN+S or click on the window you want to screen grab and hit ALT+PRINTSCREEN to capture it.