Login fails second attempt when Joomla cache is on.

Using SClogin popup modal form and login , the first attempt works but the second attempt in a short time frame with same account fails. Core joomla login function works fine. Any clue?

PHP Built On Linux N15-N17RF1 4.8.6-040806-generic #201610310831 SMP Mon Oct 31 12:33:48 UTC 2016 x86_64
Database Version 5.7.20-0ubuntu0.16.04.1
Database Collation utf8_general_ci
Database Connection Collation utf8mb4_general_ci
PHP Version 7.0.22-0ubuntu0.16.04.1
Web Server Apache/2.4.18 (Ubuntu)
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 3.8.1 Stable [ Amani ] 4-October-2017 14:00 GMT
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:56.0) Gecko/20100101 Firefox/56.0

Thanks for tip.
Yes , it works much better. No error output JINVALID_TOKEN_NOTICE as before related to checkToken().
Tested in J3.6 and J3.8.