As of 1400 UTC on January 16, 2017, ARRL Logbook of The World (LoTW) no longer will accept contacts that have been digitally signed by versions of TQSL earlier than version 2.0.
Users of earlier versions are encouraged to upgrade as soon as possible, as older TQSL versions contain uncorrected defects and display inaccurate error messages. The current versions of TQSL for Windows, OS X, and Linux are available online.
To date, LoTW has confirmed some 135 million contacts for its 90,000 users. — Thanks to Norm Fusaro, W3IZ