The document expiration has nothing to do with SQL-Ledger.
It's probably a setting in your web server. I assume that the time on the server is correct. Check your config if you use mod_expires and if there is a setting called ExpiresDefault.
If it's not on the server, it may get complicated. One of our customers regularly has such problems when he accesses our production server. This means: create an invoice, produce an error because of a wrong part number, jump back in the browser, then get a 'document expired'. We suspect that firewall of this company changes the response headers, but we are not 100 % sure.

By the way, I'd prefer if you open issues instead of discussions.