Cache problem

[Mike5783]

Hello i have an issue if anyone can help me. 
I had mybb for year now but few days ago i syddenly noticed a problem.
When a new member register at the forum i have to go in admin panel and activate their account, but recently when i activate their acount it activates but the message still stayed in admin panel at awating activation. When i click again on it, it says no new member. I have mybb - 1.8

[.m.]

are you using latest available version of MyBB (version 1.8.12) ?
looks like above referred issue is a known issue - unable to recall if it is already fixed in a recent version.

[Mike5783]

I am using version 1.8.3
As theme i am using at the moment they have not got the update for 1.8.12 yet

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Is there any solution for it without updating the version.