[HELP] Stuck at syncing, weird error (XPERIA M2)

Good morning guys,

I just started my long way to try to port Mer to the Sony Xperia M2... But I just got stuck at a point I can't even re-sync my sources anymore.

Code:

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HABUILD_SDK [eagle] linuxct@BuildUbuntu:~$ repo sync --fetch-submodules
error: in `sync --fetch-submodules`: revision refs/tags/android-5.1.1_r1 in platform/prebuilts/clang/linux-x86/3.1 not found

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I'm using THIS SOURCES, which are the ones used to build CM12.1 for my device as Galaxyfreak (who is working very hard on CM12.1 for M2) told me. In CM12.1 there are no bugs, BTW.

What do you guys think, is a problem with this sources or do I need to reset my git?