The bug was fixed by bertoost in github.
I have a similar problem with 2.2.0 with option select box.
[!+fi.tellmesomething:FormItIsSelected=`Yes`]]
Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/content/MYWEBSITE/core/cache/includes/elements/modsnippet/110.include.cache.php on line 39
Rolled back to 2.1.2 and it's ok.
In the bugtracker, this was fixed but it hasnt been included in the final code yet.
I've the fix in the code but get the same error anyway. Maybe there is something missing.
Now I do it in the way below. Not so nice but it works:
<input type="checkbox" name="agb" value="yes" [[!+fi.agb:is=`yes`:then=`checked`]] />
Best indy