Martijn,
I looked this up , it sounds as a symptom that is documented in bugID# CSCsh20354
If you have smarnet open a TAC case to confirm.
Look at your IOS version code and compare it with 1st Found-In and Known Affected Versions in bellow link.
CSCsh20354 Bug Details
client does not receive mode config data
Symptom 1: A third-party vendor VPN client may not be able to establish a VPN tunnel to a Cisco router. When you enable the debug crypto isakmp command on the Cisco router, the output shows the following:
ISAKMP:(0:4:HW:2):No IP address pool defined for ISAKMP!
ISAKMP:(0:4:HW:2):deleting SA reason "Fail to allocate ip address" state (R)
CONF_ADDR (peer x.x.x.x)
Symptom 2: Although a third-party vendor VPN client can establish a VPN
tunnel to a Cisco router, the client receives only an IP address but no DNS
configuration, split-tunnel information, or other data during the mode
configuration phase. In this situation, the debug output does not show any
errors.
Conditions: Both of these symptoms are observed only when a third-party
vendor VPN client connects to a Cisco router that functions as a VPN server.
http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCsh20354
Regards
Jorge Rodriguez