Well we can rule out the php sessions issue unless you are running multiple php versions. To be sure try this:
# start here copy line bleow me
rm -f /var/lib/lsphp/session/lsphp71/* && rm -f /var/lib/lsphp/session/lsphp72/* && rm -f /var/lib/lsphp/session/lsphp73/*
# then copy line below me
rm -f /var/lib/lsphp/session/lsphp74/* && rm -f /var/lib/lsphp/session/lsphp80/* && rm -f /var/lib/lsphp/session/lsphp81/*