Disable characters >=U+F000. These do not work properly.

Explanation from Alexey Gladkov:

The kbd value is unsigned short [1] and take a look how kernel gets
a type [2]. The last bytes are occupied by type.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/kd.h?id=06dd3dfeea60e2a6457a6aedf97afc8e6d2ba497#n103
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/keyboard.h?id=06dd3dfeea60e2a6457a6aedf97afc8e6d2ba497#n45

The problem in the kernel. In the kb_value there is no room to store such
values.

Index: kbd-2.0.4/data/keymaps/i386/qwertz/de_alt_UTF-8.map
===================================================================
--- kbd-2.0.4.orig/data/keymaps/i386/qwertz/de_alt_UTF-8.map
+++ kbd-2.0.4/data/keymaps/i386/qwertz/de_alt_UTF-8.map
@@ -160,7 +160,7 @@ shift alt keycode   2 = U+00AC  # notsig
 shift alt keycode   3 = U+201D  # right double quote
 shift alt keycode   4 = numbersign
 shift alt keycode   5 = sterling
-shift alt keycode   6 = U+FB01  # fi ligature
+#shift alt keycode   6 = U+FB01  # fi ligature
 shift alt keycode   7 = dead_circumflex
 shift alt keycode   8 = backslash
 shift alt keycode   9 = U+02DC  # small tilde
@@ -179,7 +179,7 @@ shift alt keycode  23 = Ucircumflex
 shift alt keycode  24 = Ooblique
 shift alt keycode  25 = U+220F  # n-ary product
 shift alt keycode  26 = degree
-shift alt keycode  27 = U+F8FF  # apple logo
+#shift alt keycode  27 = U+F8FF  # apple logo
 shift alt keycode  30 = Aring
 shift alt keycode  31 = Iacute
 shift alt keycode  32 = U+2122  # trade mark
@@ -188,7 +188,7 @@ shift alt keycode  34 = Igrave
 shift alt keycode  35 = Oacute
 shift alt keycode  36 = U+0131  # dotless i
 shift alt keycode  37 = U+02C6  # circumflex accent
-shift alt keycode  38 = U+FB02  # fl ligature
+#shift alt keycode  38 = U+FB02  # fl ligature
 shift alt keycode  39 = U+0152  # OE
 shift alt keycode  40 = AE
 shift alt keycode  41 = U+201C  # left double quote
@@ -210,7 +210,7 @@ control shift alt keycode   2 = U+00AC
 control shift alt keycode   3 = U+201D  # right double quote
 control shift alt keycode   4 = numbersign
 control shift alt keycode   5 = sterling
-control shift alt keycode   6 = U+FB01  # fi ligature
+#control shift alt keycode   6 = U+FB01  # fi ligature
 control shift alt keycode   7 = circumflex
 control shift alt keycode   8 = backslash
 control shift alt keycode   9 = U+02DC  # small tilde
@@ -229,7 +229,7 @@ control shift alt keycode  23 = Ucircumf
 control shift alt keycode  24 = Ooblique
 control shift alt keycode  25 = U+220F  # n-ary product
 control shift alt keycode  26 = degree
-control shift alt keycode  27 = U+F8FF  # apple logo
+#control shift alt keycode  27 = U+F8FF  # apple logo
 control shift alt keycode  30 = Aring
 control shift alt keycode  31 = Iacute
 control shift alt keycode  32 = U+2122  # trade mark
@@ -238,7 +238,7 @@ control shift alt keycode  34 = Igrave
 control shift alt keycode  35 = Oacute
 control shift alt keycode  36 = U+0131  # dotless i
 control shift alt keycode  37 = U+02C6  # circumflex accent
-control shift alt keycode  38 = U+FB02  # fl ligature
+#control shift alt keycode  38 = U+FB02  # fl ligature
 control shift alt keycode  39 = U+0152  # OE
 control shift alt keycode  40 = AE
 control shift alt keycode  41 = U+201C  # left double quote