After updating to WHMCS 8.4, 8.5. or 8.6, you may experience an ModelNotFoundException error when viewing invoices if you have enabled Client Data Snapshot and Tax Support:

Illuminate\Database\Eloquent\ModelNotFoundException: No query results for model [WHMCS\CustomField] x

Update to the 8.7.0 release at Utilities > Update WHMCS.

We have created a hotfix to resolve the error:

https://whmcs.community/files/file/186-core-17804-error-when-using-snapshot-invoice-date-for-which-a-custom-field-no-longer-exists/