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The monero-binaries at getmonero.org seem to be bugged at the moment and I can't move my funds:
internal error: Known ring does not include the spent output
The git-hub code is already fixed: https://github.com/monero-project/monero/issues/3539#issuecomment-380801093
So I build the binaries from the github code and "patched" my monero-gui download. Here is a short walkthrough, if you have the same issues:
# install dependencies
apt install build-essential cmake pkg-config libboost-all-dev libssl-dev libzmq3-dev libunbound-dev libsodium-dev git
git clone --recursive https://github.com/monero-project/monero
cd monero
# you can replace 4 with the number of cores you want to use for compiling
make -j4
# copy the new binaries to your monero-gui
cp -R build/release/bin/* /path/to/your/downloaded/monero-gui-v0.12.0.0
Now my transactions are working fine.
Stay safe.