Won't let me play Multiplayer

Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 4/3/15 11:23 AM
Description: Rendering screen

java.lang.IllegalArgumentException: bad Base64 input character at 11150: 0 (decimal)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:247)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:197)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:188)
at azl.c(SourceFile:212)
at azl.a(SourceFile:145)
at awe.a(SourceFile:25)
at awj.b(SourceFile:412)
at awj.a(SourceFile:211)
at azi.a(SourceFile:291)
at bfl.a(SourceFile:1039)
at avf.av(SourceFile:914)
at avf.a(SourceFile:325)
at net.minecraft.client.main.Main.main(SourceFile:124)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Stacktrace:
at io.netty.handler.codec.base64.Base64.decode(Base64.java:247)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:197)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:188)
at azl.c(SourceFile:212)
at azl.a(SourceFile:145)
at awe.a(SourceFile:25)
at awj.b(SourceFile:412)
at awj.a(SourceFile:211)
at azi.a(SourceFile:291)

– Screen render details –
Details:
Screen name: azi
Mouse location: Scaled: (254, 141). Absolute: (508, 197)
Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2
Stacktrace:
at bfl.a(SourceFile:1039)
at avf.av(SourceFile:914)
at avf.a(SourceFile:325)
at net.minecraft.client.main.Main.main(SourceFile:124)

– System Details –
Details:
Minecraft Version: 1.8.3
Operating System: Windows XP (x86) version 5.1
CPU: 1x Intel(R) Celeron(R) CPU 2.53GHz
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 136493144 bytes (130 MB) / 228032512 bytes (217 MB) up to 523501568 bytes (499 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.8.3
LWJGL: 2.9.4
OpenGL: Intel 865G GL version 1.3.0 - Build 4.14.10.4299, Intel
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Not using framebuffer objects because OpenGL 1.4 is not supported, EXT_blend_func_separate is supported, OpenGL 3.0 is not supported, ARB_framebuffer_object is not supported, and EXT_framebuffer_object is not supported.
Shaders are not available because OpenGL 2.1 is not supported, ARB_shader_objects is not supported, ARB_vertex_shader is not supported, and ARB_fragment_shader is not supported.
VBOs are available because ARB_vertex_buffer_object is supported.

Using VBOs: No
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: English (UK)
Profiler Position: N/A (disabled)