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It seems that this error take place when upgrading under certain circumstances, and a module or a block in the older version is newer than the module contained at the newer version upgraded to.
the newer version of Moodle may adapt a certain plugin that isn't included at the previous version standard pakage, but mostly widely used messages block for example at V2.0.0 whith is not standard at V1.9.x, and more likely for Moodle core team to adapt a certain version of the module not the last one. in this case the connotdowngrade error might happen.
what to do
simpliest action is to remove the module folder and continue upgrading