After getting a free license key (.txt file) by mail, I copy the content and enter it in the lower field of the form, then push Add. Then I get a pop-up message saying that the key is not valid. Tried that with different keys, always the same result. What do I do wrong?
Please, check that you have the latest version of the Viewer installed. Currently, it's 6.8.0.1. You can download the latest version from here https://www.remoteutilities.com/download/
If updating doesn't help, please let me know because the problem could be elsewhere (e.g. a corrupted Viewer config file).
