This doesn't seem memtest86+ issue but unexpected cases or issue with content of your /etc/default/grub (and FWIK should fail with any update launching update-grub)
From DpkgTerminalLog.txt I saw:
...
Sourcing file `/etc/default/grub'
/usr/sbin/grub-mkconfig: 10: /etc/default/grub: splash: not found
dpkg: error processing package memtest86+ (--configure):
installed memtest86+ package post-installation script subprocess returned error exit status 127
Can you post the full content of /etc/default/grub?
This doesn't seem memtest86+ issue but unexpected cases or issue with content of your /etc/default/grub (and FWIK should fail with any update launching update-grub)
From DpkgTerminalLog.txt I saw: grub-mkconfig: 10: /etc/default/grub: splash: not found
...
Sourcing file `/etc/default/grub'
/usr/sbin/
dpkg: error processing package memtest86+ (--configure):
installed memtest86+ package post-installation script subprocess returned error exit status 127
Can you post the full content of /etc/default/grub?