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I'm having the very same issue on the same modpack! I also have Optifine but I messed around with settings and nothing Optifine-related seemed to fix it. I also messed with the in-game config files of OpenBlocks to see if anything made a difference and it didn't. I'd love if someone could shed light on this issue!
My only advice would be to disable Optifine completely and see if it has any effect. I know some people swear by it, but it has a long history of varying degrees of incompatibility with Forge.