Hi, Philipp-
The errts file is a residual from regression modeling. It should have mean zero. During regression, the baseline is modeled, and the baseline typically includes the mean value of the data.
When using the 'scale' block, the data prior to the regress block are scaled to have mean=100 and a (generously) capped range of [0,200]. This scaling is done on per voxel basis (as opposed to something like grand mean scaling, which is volume-wide), and so it creates a dataset that has the interpretation of "local BOLD % change" at each time series; the residuals have these units. This seems like a quantity with meaningful units that is as comparable as possible from resting state FMRI data.
--pt
