A user got the message: "The trial period of this application has been manipulated". What can I do to let him run my application again?

You can do any of the following options:

1) If you have enabled "Trial Extension" (In Trial Settings panel) before protecting your application, you can send a trial extension key to your customer.

2) If you haven’t enabled the "Trial Extension" option, you can reprotect your application enabling that option and send him a trial extension key.

3) You can reprotect your application with a new "Trial Unique Key" (In Software panel). By doing that, your new protected application will have a fresh trial (as if the trial was reset on your customer’s PC).

4) You can send him an expiry license key, though this option is not suitable if your application works in different ways in trial and registered mode.