That doesn't make sense. The data stored (and truncated) in the cache 
is obviously a serialized iCalendar object. So this cannot be a 
problem with retrieving the calendar.
Second, we catch any errors from retrieving remote calendars.
Finally, we save data into the cache *immediately* after serializing 
it. So either you PHP's serialization is broken, or something in the 
cache code is truncating the data, or there is some issue with your 
database. Remote calendars can be very big, are you sure that you are 
not hitting any database limit, like MySQL's max_allowed_packet.

exact message is:

Truncated message: chunk size 4096 exceeds remaining data size 1358 at 
pos 119022 of 120380

oh i overlooked that report where i shoulg give feedback, data is not 
stored completly in cache, because the request times out i think 
because sometimes i get instead of the empty error message a message 
that indicates that getting more bytes from resource fails (tries to 
read 4096 bytes got only 3000). when i see it the next time i post the 
exact wording

That still doesn't answer my question.

when i turn off the caching (set explizit $cache = false as first 
statement) in getRemoteCalendar()) now it works, but is slower

ok last comment wasn't right about the http pecl extension, with or 
without it makes no difference

i disabled the http pecl extension now it works, data was not stored 
completly in the cache before

Please take a look at the cache storage backend to see if this value 
is broken in the storage too, or if this happens when reading from 
there.

i did a output to a textfile before the unserialize call; i think the 
entry is not completly written to the cache or it is not completly 
retrieved from it. last bytes of the file are:

{s:4:"name";s:8:"LOCATION";s:6:"params";a:0:{}s:5:"value";s:15:"Halle/Westfalen";s:6:"values";a:1:{i:0;s:15:"Halle/Westfalen";}}i:21;a:4:{s:4:"name";s:8:"SEQUENCE";s:6:"params";a:0:{}s:5:"value";i:0;s:

i have no idea whats going on here

The question is why you have invalid serialized data in your cache. 
What's the content of $calendar before that line from the error message?

i have set up a remote calender (google calender), when i now open 
kronolith, i got a line under the menu bar with a red x and no message 
and all entries of the remote calender are missing, in my log file i 
found these message:

2011-04-26T13:17:23+02:00 NOTICE: HORDE [kronolith] PHP ERROR: 
unserialize(): Error at offset 312097 of 312107 bytes [pid 27443 on 
line 521 of "/var/www/horde4/kronolith/lib/Driver/Ical.php"]
2011-04-26T13:17:45+02:00 NOTICE: HORDE [kronolith] PHP ERROR: 
unserialize(): Error at offset 312097 of 312107 bytes [pid 27839 on 
line 521 of "/var/www/horde4/kronolith/lib/Driver/Ical.php"]
2011-04-26T13:19:05+02:00 NOTICE: HORDE [kronolith] PHP ERROR: 
unserialize(): Error at offset 312097 of 312107 bytes [pid 27583 on 
line 521 of "/var/www/horde4/kronolith/lib/Driver/Ical.php"]

eighter there needs to be a good error message or something other to 
resolve the unserialize