Coupon Login error with Mikro tik

Hi,

Today I have faced some new issue with my Mikro-tik which was integrated with my radius server.

I had a coupon which was created in the radius server it’s 24 hrs coupons which validity for this coupon will 24 hrs.
I have logged with this coupon, then I successfully logged in & i got internet also. i have worked around two hours after that suddenly i got the login page again.

I have tried the same coupon to login again, I got the attached error.

What could be reason behind this!!! How to avoid in future!!!
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