[Touch Pro2, Tilt 2 Android General] adlerweb:
So - think i found the problem... First some radio-logs:

Info+:

logcat -b radio *:I
I/RIL ( 1293): Opening tty device /dev/smd0
I/PHONE ( 1392): Network Mode set to 0
I/PHONE ( 1392): Cdma Subscription set to 1
I/RILJ ( 1392): Connected to 'rild' socket
I/RILC ( 1293): libril: new connection
I/RILC ( 1293): RIL Daemon version: HTC Vogue Community RIL 0.8 [mdrobnak-1]
W/GSM ( 1392): Can't open /system/etc/voicemail-conf.xml
W/GSM ( 1392): Can't open /system/etc/spn-conf.xml
I/PHONE ( 1392): Creating GSMPhone
E/GSM ( 1392): Wrong network type: 0
E/GSM ( 1392): Wrong network type: 0
E/RIL ( 1293): requestOperator must not return error when radio is on
E/RIL ( 1293): requestQueryNetworkSelectionMode must never return error when radio is on
I/RIL ( 1293): Calls=0,Valid=0
I/RIL ( 1293): Audio Close
E/RILC ( 1293): RIL_onRequestComplete: invalid RIL_Token
E/GSM ( 1392): RIL implementation has returned an error where it must succeedcom.android.internal.telephony.CommandException: GENERIC_FAILURE
W/GSM ( 1392): error parsing RegistrationState: java.lang.NumberFormatException: unable to parse 'ffffffff' as integer
E/GSM ( 1392): RIL implementation has returned an error where it must succeedcom.android.internal.telephony.CommandException: GENERIC_FAILURE
E/GSM ( 1392): Wrong network type: 0
E/GSM ( 1392): Wrong network type: 0
E/RIL ( 1293): requestOperator must not return error when radio is on
E/RIL ( 1293): requestQueryNetworkSelectionMode must never return error when radio is on
E/RIL ( 1293): requestOperator must not return error when radio is on
E/RIL ( 1293): requestQueryNetworkSelectionMode must never return error when radio is on
E/GSM ( 1392): RIL implementation has returned an error where it must succeedcom.android.internal.telephony.CommandException: GENERIC_FAILURE
W/GSM ( 1392): error parsing RegistrationState: java.lang.NumberFormatException: unable to parse 'ffffffff' as integer
E/GSM ( 1392): RIL implementation has returned an error where it must succeedcom.android.internal.telephony.CommandException: GENERIC_FAILURE
E/GSM ( 1392): Wrong network type: 0
E/GSM ( 1392): Wrong network type: 0
E/GSM ( 1392): Exception querying IMSI, Exception:com.android.internal.telephony.CommandException: GENERIC_FAILURE
E/GSM ( 1392): Wrong network type: 0
E/GSM ( 1392): Wrong network type: 0
E/RIL ( 1293): ERROR: requestGetPreferredNetworkType() failed - modem does not support command

Interesting in debug:

D/AT ( 1293): AT> AT+CGDCONT?
D/AT ( 1293): AT< +CGDCONT: 1,"IP","internet.eplus.de","66.251.138.10",0,0
D/AT ( 1293): AT< +CGDCONT: 2,"IP","internet.eplus.de","",0,0
D/AT ( 1293): AT< OK
D/RILJ ( 1392): [UNSL]< UNSOL_DATA_CALL_LIST_CHANGED [DataCallState: { cid: 1, active: 0, type: IP, apn: internet.eplus.de, address: 66.251.138.10 }, DataCallState: { cid: 2, active: 0, type: IP, apn: internet.eplus.de, address: }]
D/RILJ ( 1392): [UNSL]< UNSOL_DATA_CALL_LIST_CHANGED [DataCallState: { cid: 1, active: 0, type: IP, apn: internet.eplus.de, address: 66.251.138.10 }, DataCallState: { cid: 2, active: 0, type: IP, apn: internet.eplus.de, address: }]
D/GSM ( 1392): GSMDataConnTrack handleMessage { what=26 when=93050 obj=android.os.AsyncResult@43f56c40 }
D/GSM ( 1392): [GsmDataConnectionTracker] ***trySetupData due to gprsAttached
D/GSM ( 1392): [DSAC DEB] trySetupData with mIsPsRestricted=false
D/GSM ( 1392): [GsmDataConnectionTracker] trySetupData: Not ready for data: dataState=IDLE gprsState=0 sim=false UMTS=true phoneState=IDLE isDataAllowed=true dataEnabled=true roaming=false dataOnRoamingEnable=true ps restricted=false desiredPowerState=true MasterDataEnabled=true
D/GSM ( 1392): GSMDataConnTrack handleMessage { what=6 when=93160 obj=android.os.AsyncResult@43f58168 }
D/GSM ( 1392): GSMDataConnTrack handleMessage { what=6 when=93160 obj=android.os.AsyncResult@43f59080 }
D/GSM ( 1392): VM: ADN Record 'Mailbox' '+491779911 null' EF[MBDN]
D/GSM ( 1392): SIMRecords: record load complete
D/GSM ( 1392): [IccCard] Broadcasting intent ACTION_SIM_STATE_CHANGED LOADED reason null
D/GSM ( 1392): GSMDataConnTrack handleMessage { what=4 when=93350 obj=android.os.AsyncResult@43f5e700 }
D/GSM ( 1392): [GsmDataConnectionTracker] No APN found for carrier: null
D/GSM ( 1392): [GsmDataConnectionTracker] setState: FAILED
D/GSM ( 1392): [GsmDataConnectionTracker] Clean up connection due to null
D/GSM ( 1392): [GsmDataConnectionTracker] setState: DISCONNECTING
D/GSM ( 1392): [GsmDataConnectionTracker] cleanUpConnection: !tearDown, call conn.resetSynchronously
...
D/AT ( 1293): AT> AT+CGDCONT?
D/AT ( 1293): AT< +CGDCONT: 1,"IP","internet.eplus.de","66.251.138.10",0,0
D/AT ( 1293): AT< +CGDCONT: 2,"IP","internet.eplus.de","",0,0
D/AT ( 1293): AT< OK
...
D/GSM ( 1392): Poll ServiceState done: oldSS=[0 home E-Plus Mobilfunk E-Plus 26203 UMTS CSS not supported -1 -1RoamInd: -1DefRoamInd: -1EmergOnly: false] newSS=[0 home E-Plus Mobilfunk E-Plus 26203 UMTS CSS not supported -1 -1RoamInd: -1DefRoamInd: -1EmergOnly: false] oldGprs=0 newGprs=0 oldType=UMTS newType=UMTS

So as it seems the phone connects to the APN itself but after the SIM-card is initialized it suddenly forgets which operator its associated to and does not establish the actual data connection. (Operator = null). There is a Post on XPERIA Mobile Linux blog which describes the whole problem very detailed. Its - as already guessed - a timing issue: MMC/MNC are devirated from the IMSI which comes from the SIM - strangely the sim says its ready even if its still initializing. The IMSI-Request fails, MMC/MNC are NULL and foobar happens. His patch however showed no effect here (maybe just something wrong with my build or i overlooked something but i guess hardware/xdandroid-ril/htcgeneric-ril.c is the right place to fix the root cause). Since im not that good in C and couldn't really understand why my changes showed no effect i finally stopped debugging for now, took my coding-sledgehammer and changed getSIMOperatorNumeric in frameworks/base/telephony/java/com/android/internal/telephony/gsm/SIMRecords.java (~449) to return "26203"; bypassing the whole SIM-Card/IMSI-readout.