Resolved Export Order "since Last Run" Error 500 - DateTime given!

Hi,
with the ultimate version 9.1.0 when run the export of the order of my virtuemart ecommerce, obtain the following error:

Status error: 500
Status message: error
{"error":true,"code":0,"message":"substr(): Argument #1 ($string) must be of type string, DateTime given"}

My template have a setting option "data Range" = since last run

if I try to change data rage with any other parameter, the problem does not occur

In previous version (8.22.1) the error is not present!!!

Can you help me?
This is a BUG?

Many Thanks.