[horde] kronolith uograde error

[Alexandru Timohi]

i've just upgraded my Horde3 configuration and now i got horde
3.2+Kronolith3.2+Nag3.2+Turba2.1.4+Imp4.1.4+passwd3.0.1

everything works just fine, but there is still an error when i access
calendar
Notice: unserialize(): argument is not a string in
/srv/www/htdocs/horde3/kronolith/lib/Kronolith.php on line 563

but the kronolith works fine

the question is what causes that error message?