please help on connection to wifi after updates

Jan 16, 2013
it is most gratefule for me that the kind members in this forum did previously help me to resolve a problem on my mi2. now i have another serious issue with my mi 2. so i come here with request for help again. i did search the web for possible solution and found none. there is a related but not the same problem posted in this forum. but the problem there is not exactly the same as mine: more importantly it has been abandoned for quite some time.
here is the problem: i use the rom that came with the phone for several months without any issues (i never tempered with the rom at all; not even bother to update it). but since two weeks ago i updated to miui jlb 12.0 and then jlb 15.0 on my mi2 (unroot) , i've been unable to connect to a wifi secured with wep. it can see the network just fine, and lets me put in a password. then it says, "saved," but will not connect; not even shows: "acquiring ip ..." as usual as it previously did. this is a wifi secured with wep. my phone currently can still connect to another open source wifi (as it did before the updates). my rooted (gingerbread) sony xperia play can and still be able to conect to both the same secured and open wifi, the problem should therefore unrelate to the routers or the soueces of the wifi. i tried to solve this problem according to several suggested-solutions e.g. delete-and re-enter the wifi, using wifi manager app for fixing etc. all failed. one suggestion in a related post herein this forum said that the dhcp should be deleted from the phone; but i cannot locate (see no such file) dhcp in the data folder.
any suggestions to solve this wifi problem would be greatly appreciated.