I usually don't have this affect when installing mods like Optifine, however sometimes they just fail to completely install.

It's a longshot but perhaps utilising a texture pack will fix the problem.

Otherwise try reinstalling Minecraft and the mods - remove all files under .minecraft from %APPDATA%, just remember to copy over your saves. Once Minecraft has downloaded all the files it needs, re-install your mods.

If the chunk is near the spawn, it's probably spawn protection (See: server.properties) and the player that was interacting with objects in the chunk was a person of a higher rank (in vanilla, this would be an operator).

Determines the radius of the spawn protection. Setting this to 0 will not disable spawn protection. 0 will protect the single block at the spawn point. 1 will protect a 3x3 area centered on the spawn point. 2 will protect 5x5, 3 will protect 7x7, etc. This option is not generated on the first server start and appears when the first player joins. If there are no ops set on the server, the spawn protection will be disabled automatically.

The default protection radius is 16x16x16, which would explain why it is 1 chunk.