Occasional failing of ^SWWAN on PLS8-E | Telit Cinterion IoT Developer Community

Hi,
we us a PLS8-E (or PLS8-US, region dependent) in our small data loggers for data upload. For every upload-session we power on the modem, configure, upload and shut down. We do this some years without problems.
Then we wanted to get rid of the pppd overhead and switched to using the AT^SWWAN functionality.
In 95% the procedure works, but occasionally the AT^SWWAN=1 fails:

AT+CPIN?
+CPIN: READY
OK

ATI
Cinterion
PLS8-E
REVISION 03.017

AT+CEMODE? // We make sure, the device is datacentric
+CEMODE: 2
OK

AT+COPS=2 // deregister from the network to allow PDP 1 to be changed
OK

AT+CGDCONT=1,"IP","internet.telekom","0.0.0.0",0,0
OK

AT+COPS=0 // register again
OK

AT^SGAUTH=1,0 // no user/pass for this APN/provider required
OK

AT^SWWAN=1,1,1 // activate USB network interface
+CME ERROR: unspecified GPRS error

In this situation, a delay and repeat of the last command also fails.

Without changing anything I then continue with ATD*99***1# to build the PPP connection with pppd. This always works, so PDP 1 is ok, the modem can connect to the network etc. Why did SWWAN fail?

What might be the reason for the occasional "+CME ERROR: unspecified GPRS error"?
With the next power off/power on, the SWWAN works perfectly again. Same network, same SIM, only minutes later.

The command sequence is trimmed to fit as many different SIM cards as possible, which may differ in PDP usage etc.

Is there a state which I should check before issuing the AT^SWWAN=1,1,1?

TIA,
Wolfram