Access denied to offline encrypted files

I recently experienced some issues on my laptop running Vista Business.
I use a couple of shared folders from the file server, which are offline
available on the laptop.
Since 2 months, some files inside these offline folders become inaccessable
on the laptop. I get the error message 'Access denied' when I try to open
them again. On my desktop PC I have no problems accessing these files, using
the same domain account. (sometimes files that I worked on yesterday are
'denied' the next day)

Trying to solve the issue, I disabled the option 'offline available' of one
of the shared folders on the laptop. As soon as I connected to the laptop to
the network, the files on the share which where not accessable before, opened
without problems.

Later, I made another change: I disabled the option 'encrypt offline files'.
This also solved the problem, still having the files offline available. All
offline available files previously giving 'access denied' worked again! So
with 'encrypt offline files' disabled, everything runs smoothly, but I rather
have the files encrypted. I already checked the share and NTFS rights on the
server, this is not the problem. Installing Vista Service pack 1 did not
change the behaviour.

Does anyone have an idea what may cause this 'Access denied' issue?