First the Crash Report:
---- Minecraft Crash Report ----
// My bad.

Time: 26.12.14 21:22
Description: Ticking screen

java.lang.IndexOutOfBoundsException
at java.nio.Buffer.checkIndex(Buffer.java:540)
at java.nio.DirectIntBufferU.get(DirectIntBufferU.java:253)
at bom.a(SourceFile:271)
at bom.a(SourceFile:220)
at ban.a(SourceFile:1694)
at ban.a(SourceFile:1652)
at bln.a(SourceFile:123)
at hd.a(SourceFile:70)
at hd.a(SourceFile:13)
at ej.a(SourceFile:174)
at bct.c(SourceFile:78)
at ban.p(SourceFile:1343)
at ban.ak(SourceFile:774)
at ban.f(SourceFile:728)
at net.minecraft.client.main.Main.main(SourceFile:148)

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

– Head –
Stacktrace:
at java.nio.Buffer.checkIndex(Buffer.java:540)
at java.nio.DirectIntBufferU.get(DirectIntBufferU.java:253)
at bom.a(SourceFile:271)
at bom.a(SourceFile:220)
at ban.a(SourceFile:1694)
at ban.a(SourceFile:1652)
at bln.a(SourceFile:123)
at hd.a(SourceFile:70)
at hd.a(SourceFile:13)
at ej.a(SourceFile:174)
at bct.c(SourceFile:78)

– Affected screen –
Details:
Screen name: bct

– Affected level –
Details:
Level name: MpServer
All players: 0 total; []
Chunk stats: MultiplayerChunkCache: 0, 0
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (8,64,8), Chunk: (at 8,4,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 0 game time, 0 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Forced entities: 0 total; []
Retry entities: 0 total; []
Server brand: ~_ERROR~ NullPointerException: null
Server type: Non-integrated multiplayer server
Stacktrace:
at blr.a(SourceFile:289)
at ban.b(SourceFile:1972)
at ban.f(SourceFile:737)
at net.minecraft.client.main.Main.main(SourceFile:148)

– System Details –
Details:
Minecraft Version: 1.7.9
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 82171936 bytes (78 MB) / 179937280 bytes (171 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.7.9
LWJGL: 2.9.1
OpenGL: GeForce GTX 970/PCIe/SSE2 GL version 4.4.0 NVIDIA 344.75, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Anisotropic filtering is supported and maximum anisotropy is 16.
Shaders are available because OpenGL 2.1 is supported.

Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: Deutsch (Deutschland)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Anisotropic Filtering: Off (1)

My Problem:

I cant login to a server on the version 1.7.9 but i can join other servers on the version 1.8.

I look forward heaing from you.
Thank you

Luka