I have a Vigor 2862Lac running firmware version 3.9.6.1_MDM3.

Prior to the router's unexpected reboot a few minutes ago, every user apart from our MD could connect from their iPhone over mobile data using IKEv2. For our MD, every connection attempt triggered a "Can't decrypt message" entry in the SysLog. After so many attempts and during the working day, the router eventually decided to reboot itself and logged a firmware exception.

Following the unexpected reboot, everyone including our MD can now connect to VPN from their iPhones over mobile data using IKEv2.

It makes no sense why a reboot should temporarily fix this issue, and I say temporary because this issue has happened previously using this firmware version and others, so I am expecting the same issue to reoccur at some point.

Has anyone else experienced this issue? Is anyone aware of a fix, even if that involves downgrading the firmware?

The firmware exception log is as follows:

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firmware exception debug info

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EPC=1100fc03, Cause=6572742d, Status=837e3548, BadAddr=00000000, RetAddr=805780 08
exception id=11(CpU: Coprocessor unusable)
exception occured in IdleTask Timeout
task_id=133, SP=0x837e33f8(3958 bytes free, 2584 used bytes)
system tick =379278323
CURRENT_VERSION : 3.9.6.1_R
get from address : 80004000
Dump CPU registers:

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$ 0: 00000000 fffffffe 00001995 82e942e0
$ 4: 00000000 82f3d828 82f3d8d0 82e943f8
$ 8: 837e35d0 000000a1 0a6d8d80 2c205065
$12: 00000000 00000023 00000000 5eee6cdd
$16: 837e3558 837e3548 00000023 837e35c8
$20: 837e35c0 82ec0000 82eba29c 00000001
$24: 00000010 00000000 00000000 00000000
$28: 00000000 837e33f8 80e40000 80578008
List a possible caller functions:

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func( 0)=80578008
func( 1)=80e40000
func( 2)=80597e14
func( 3)=81152e10
func( 4)=811da824
func( 5)=81147754
func( 6)=8055a25c
func( 7)=80e40484
func( 8)=80e3fb80
func( 9)=80e40000
func(10)=8057f7b8
func(11)=8119e0f8
func(12)=80e40484
func(13)=8115c894
func(14)=80e416e8
func(15)=8059c278
func(16)=8055f484
func(17)=8055f180
func(18)=8055f4e8
List backtrace functions:

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func( 0)=1100fc03