Download remount in play store (or better yet, download "mount /system " in play store) , choose the /cust folder as rw, click on the title.
Remounting worked indeed! Problem fixed. Thanks rurou a lot.
Summarizing, my "root problem" was in reality "writable mount problem" (as I finally suspected). Remount app is very helpful for users with limited linux experience. I hope this info will help another users who had problems with root access.
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