If I do a fsck (with unmount first) on the webGUI, the fsck action is indeed done (it takes a while to check almost 900GB of stored data in my case), but I never see the result text of the fsck action on the webGUI. Only the first line is shown. In the log file there is some related error about this. If I do fsck with the CLI it's no problem at all and all the info is shown.
The funny thing is, if there is much less stored data on the graid5 array (say less then ~500GB) the result of the fsck action is indeed showing on the webGUI. (It takes lesser time to check the disk array and the webGUI is updated correctly)
It looks like it is taking to long for the webpage script to update it's self. (bug?)