Update InstallationFAQ.md

Guides/InstallationIssues.md

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-# FAQ
+# Installation issues
+
+## If OEM-unlock code doesn't work
+
+If you get the message "Password verification failed" on your phone after entering the unlock code.
+It might be the problem, that you are using an old Android version. So update your Android and try it again.
+
+For additional information see here: https://forum.fairphone.com/t/bootloader-unlocking-code-for-fairphone-3-not-working/66685/6
+
 
 ## If "5. Install LinegaeOS recovery image" does not work
 
 You an try `fastboot flash boot_a <image_filename.img>` or `fastboot flash boot_b <image_filename.img>`. The `boot_a` is the name of our recovery boot partition. If your FP was rooted several times this my result in a non factory partition name ...
 
 With `fastboot getvar current-slot` you can find out whether your suffix should be `_a` or `_b`. Find more possible names at https://forum.fairphone.com/t/find-out-boot-partitions-name/77171
+