I'm running trunk Redmine (r8903 as we speak) with ruby 1.9.3 and Rails 2.3.14.
If logging in with a user who is authenticated through LDAP, I get an "incompatible encoding" error.
Upgrading to net-ldap-0.3.1 solves the problem.

Log is as follows:

Processing AccountController#login (for 192.168.0.154 at 2012-02-18 14:48:27) [POST]
Parameters: {"authenticity_token"=>"[FILTERED]", "back_url"=>"[FILTERED]", "username"=>"[FILTERED]", "password"=>"[FILTERED]", "login"=>"[FILTERED]", "controller"=>"account", "action"=>"login"}

Encoding::CompatibilityError (incompatible character encodings: UTF-8 and ASCII-8BIT):
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ber/core_ext/array.rb:61:in join'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ber/core_ext/array.rb:61:into_ber_seq_internal'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ber/core_ext/array.rb:43:in to_ber_appsequence'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ldap.rb:1374:inblock in search'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ldap.rb:1361:in loop'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ldap.rb:1361:insearch'
/usr/local/lib/ruby/gems/1.9.1/gems/net-ldap-0.2.2/lib/net/ldap.rb:635:in search'
app/models/auth_source_ldap.rb:110:inget_user_dn'
app/models/auth_source_ldap.rb:37:in authenticate'
app/models/user.rb:142:intry_to_login'
app/controllers/account_controller.rb:149:in password_authentication'
app/controllers/account_controller.rb:144:inauthenticate_user'
app/controllers/account_controller.rb:31:in login'
<internal:prelude>:10:insynchronize'
thin (1.3.1) lib/thin/connection.rb:80:in block in pre_process'
thin (1.3.1) lib/thin/connection.rb:78:incatch'
thin (1.3.1) lib/thin/connection.rb:78:in pre_process'
thin (1.3.1) lib/thin/connection.rb:53:inprocess'
thin (1.3.1) lib/thin/connection.rb:38:in receive_data'
eventmachine (0.12.10) lib/eventmachine.rb:256:inrun_machine'
eventmachine (0.12.10) lib/eventmachine.rb:256:in run'
thin (1.3.1) lib/thin/backends/base.rb:61:instart'
thin (1.3.1) lib/thin/server.rb:159:in start'
thin (1.3.1) lib/thin/controllers/controller.rb:86:instart'
thin (1.3.1) lib/thin/runner.rb:185:in run_command'
thin (1.3.1) lib/thin/runner.rb:151:inrun!'
thin (1.3.1) bin/thin:6:in <top (required)>'
/usr/local/bin/thin:19:inload'
/usr/local/bin/thin:19:in `