Updating of the playback history db can randomly fail & potentially is due to a mis-match of the 2 files involved from prior build stability issues.
When WACUP is not running, run wacup.exe /compacthistory from a command-line prompt within the WACUP program folder might help to correct the issue.
Otherwise removing the current history.[dat|idx] files from the WACUP settings folder (typically found in %appdata%\wacup\plugins\ml or go to Preferences -> Advanced (is at the bottom of the tree) -> Diagnostics -> Settings Locations) is for now the only alternative & that will cause any playback history to be lost.
Some modern skins may not draw correctly with either partial or completely missing window frames or flashing elements. This seems to be due to a change made on Feb 20th in the interim / beta builds to resolve seeker bar elements along with some other elements not showing as expected in the skins. A solution to keep the prior fix & un-break the skins now being reported as broken is being looked into.