Second X-Server fails to start: BUG: triggered 'if (axnum >= dev->valuator->numAxes)'

I'm using Xubuntu 15.04, installed from scratch with the default Xorg package (7.7) and kernel 3.19.0.

The first X-Server starts fine. I'm actually writing this report under it right now.

But when I try to launch another X-Server from a text console (using the command 'startx -- :1'), this happens:

[ 6222.756] (II) XINPUT: Adding extended input device "Logitech Gaming Mouse G600" (type: KEYBOARD, id 11)
[ 6222.756] (**) Option "xkb_rules" "evdev"
[ 6222.756] (**) Option "xkb_model" "pc105"
[ 6222.756] (**) Option "xkb_layout" "de"
[ 6222.756] (**) Option "xkb_variant" "nodeadkeys"
[ 6222.756] (EE) BUG: triggered 'if (axnum >= dev->valuator->numAxes)'
[ 6222.756] (EE) BUG: ../../Xi/exevents.c:2087 in InitValuatorAxisStruct()
[ 6222.756] (EE)
[ 6222.756] (EE) Backtrace:
[ 6222.757] (EE) 0: /usr/bin/X (xorg_backtrace+0x56) [0x7ff970fed556]
[ 6222.757] (EE) 1: /usr/bin/X (InitValuatorAxisStruct+0x67) [0x7ff970f7e8f7]
[ 6222.757] (EE) 2: /usr/lib/xorg/modules/input/evdev_drv.so (0x7ff9682b4000+0x4f68) [0x7ff9682b8f68]
[ 6222.757] (EE) 3: /usr/lib/xorg/modules/input/evdev_drv.so (0x7ff9682b4000+0x5406) [0x7ff9682b9406]
[ 6222.757] (EE) 4: /usr/lib/xorg/modules/input/evdev_drv.so (0x7ff9682b4000+0x7088) [0x7ff9682bb088]
[ 6222.757] (EE) 5: /usr/bin/X (ActivateDevice+0x4a) [0x7ff970e870fa]
[ 6222.757] (EE) 6: /usr/bin/X (0x7ff970e3a000+0xa5a81) [0x7ff970edfa81]
[ 6222.757] (EE) 7: /usr/bin/X (0x7ff970e3a000+0xbbb8b) [0x7ff970ef5b8b]
[ 6222.757] (EE) 8: /usr/bin/X (0x7ff970e3a000+0xbc173) [0x7ff970ef6173]
[ 6222.757] (EE) 9: /usr/bin/X (config_init+0x9) [0x7ff970ef4e99]
[ 6222.757] (EE) 10: /usr/bin/X (InitInput+0xbb) [0x7ff970ed313b]
[ 6222.757] (EE) 11: /usr/bin/X (0x7ff970e3a000+0x5c259) [0x7ff970e96259]
[ 6222.757] (EE) 12: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf0) [0x7ff96eb20a40]
[ 6222.757] (EE) 13: /usr/bin/X (0x7ff970e3a000+0x4662e) [0x7ff970e8062e]
[ 6222.757] (EE)

I think it's trying to initialize the additional keys on my mouse as keyboard device and has some problems with that.

These errors do not occur on the first X-Server, only the second. I found a few reports on this via Google. But those were other instances and none of them had a solution anyway.

Before I installed 15.04, I was running 14.04 and there the second X-Server working without problems.

The full Xorg error log is attached to this post.