I doubt there is a problem with mapchooser and CSS otherwise there would be tons of reports about it. You could try disabling everything but MM:S, SM, and SM's first-party extensions and plugins to see if it makes a difference.
Code:
version
Protocol version 19
Exe version 1.0.0.68 (cstrike)
Exe build: 11:59:31 Nov 2 2011 (4743) (240)
sm version
SourceMod Version Information:
SourceMod Version: 1.4.0
SourcePawn Engine: SourcePawn 1.1, jit-x86 (build 1.4.0)
SourcePawn API: v1 = 4, v2 = 3
Compiled on: Oct 28 2011 18:00:59
Build ID: 3425:ddd707c3454c
http://www.sourcemod.net/
sm plugins list
[SM] Listing 19 plugins:
01 "Rock The Vote" (1.4.0) by AlliedModders LLC
02 "Fun Commands" (1.4.0) by AlliedModders LLC
03 "Basic Votes" (1.4.0) by AlliedModders LLC
04 "Fun Votes" (1.4.0) by AlliedModders LLC
05 "Admin File Reader" (1.4.0) by AlliedModders LLC
06 "Basic Comm Control" (1.4.0) by AlliedModders LLC
07 "Reserved Slots" (1.4.0) by AlliedModders LLC
08 "Anti-Flood" (1.4.0) by AlliedModders LLC
09 "Admin Help" (1.4.0) by AlliedModders LLC
10 "Sound Commands" (1.4.0) by AlliedModders LLC
11 "Nextmap" (1.4.0) by AlliedModders LLC
12 "Admin Menu" (1.4.0) by AlliedModders LLC
13 "Player Commands" (1.4.0) by AlliedModders LLC
14 "Basic Commands" (1.4.0) by AlliedModders LLC
15 "Basic Chat" (1.4.0) by AlliedModders LLC
16 "Basic Info Triggers" (1.4.0) by AlliedModders LLC
17 "Client Preferences" (1.4.0) by AlliedModders LLC
18 "Basic Ban Commands" (1.4.0) by AlliedModders LLC
19 "MapChooser" (1.4.0) by AlliedModders LLC