Hello,
what is the problem? How can I fix it?

Strict Standards: Only variables should be assigned by reference in /data/web/XXXXXXXXXX/html/modules/mod_slitslider/mod_slitslider.php on line 16
Strict Standards: Non-static method JVersion::isCompatible() should not be called statically in /data/web/XXXXXXXXXX/html/modules/mod_slitslider/tmpl/default.php on line 26

System info:
Joomla! 3.6.2 Stable
PHP 5.4.45-0+deb7u4

Slitslider version please!

Sorry.

SlitSlider
12/Dez/2013 - Version 1.0.002 - Some bug fixed
21/Nov/2013 - Version 1.0.001 - Main Release
The Slit Slider Joomla module is a slideshow with a twist:...

jp-xtra 1.0.003
Warp Framework 7.3.1

I updated the Warp Framework 7.3.28 and widgekit 1.5.9 July 2016, but I can't see change. Error:

Strict Standards: Only variables should be assigned by reference in XXXXXXXXXX/html/modules/mod_slitslider/mod_slitslider.php on line 16
Strict Standards: Non-static method JVersion::isCompatible() should not be called statically in XXXXXXXXXX/html/modules/mod_slitslider/tmpl/default.php on line 26

For this reason we do not use the Slitslider anymore.
The current Xtra theme have no slitslider included! Where did you found the Theme and when did you purchased the theme?
demo.joomlaplates.com/jp_xtra/

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And its not an error its just a PHP notice.
You can disable the notice in Joomla System configuration > Server > Error messages > minimal or OFF