Comment # 11 on bug 1088067 from Martin Vidner

With this helper in ruby-bindings Ops.add:

diff --git a/src/ruby/yast/ops.rb b/src/ruby/yast/ops.rb
index 84be7bf..bc9cee0 100644
--- a/src/ruby/yast/ops.rb
+++ b/src/ruby/yast/ops.rb
@@ -242,6 +242,8 @@ module Yast
       else
         first + second
       end
+    rescue => e
+      raise e.class, e.message + " (#{first.inspect}, #{second.inspect})"
     end

     # Subtracts second from first.

We get:
2018-04-05 14:30:01 <3> install(3904) [Ruby] yast/wfm.rb:253 Client
/mounts/mp_0001/usr/share/YaST2/clients/inst_disk_proposal.rb failed with
'incompatible character encodings: ASCII-8BIT and UTF-8
("<ul><li>\xD0\xA1\xD0\xBE\xD0\xB7\xD0\xB4\xD0\xB0\xD1\x82\xD1\x8C GPT
\xD0\xBD\xD0\xB0 /dev/sda</li><li>Create partition /dev/sda1 (8.00
\xD0\x9C\xD0\xB8\xD0\x91) as BIOS Boot Partition</li><li>Create partition
/dev/sda2 (40.00 \xD0\x93\xD0\xB8\xD0\x91) for / with btrfs</li><li>Create
partition /dev/sda3 (117.99 \xD0\x93\xD0\xB8\xD0\x91) for /home with
xfs</li><li>Create partition /dev/sda4 (2.00 \xD0\x93\xD0\xB8\xD0\x91) for
swap</li><li>", "���������������� �� ���������������� 9 (<a
href=\"actions_presenter--subvolumes\">���������������� ����������������������</a>)")'
(Encoding::CompatibilityError).
Backtrace:
/usr/lib64/ruby/vendor_ruby/2.5.0/yast/ops.rb:246:in `rescue in add'
/usr/lib64/ruby/vendor_ruby/2.5.0/yast/ops.rb:230:in `add'
/mounts/mp_0001/usr/share/YaST2/modules/HTML.rb:139:in `block in List'

The maddening thing is that the first string DOES represent a valid UTF-8
string, "<ul><li>�������������� GPT ���� /dev/sda</li><li>Create partition /dev/sda1
(8.00 ������) as BIOS Boot Partition</li><li>Create partition /dev/sda2 (40.00
������) for / with btrfs</li><li>Create partition /dev/sda3 (117.99 ������) for /home
with xfs</li><li>Create partition /dev/sda4 (2.00 ������) for swap</li><li>"

So my hypothesis about a single misencoded string seems wrong.