Cannot enter Ads Places

I am maintaining 2 WordPress sites. One is the primary in another remote site and another locally as back up server. Both are on 2.5.94. On primary server running ok although entering Ads Places nonetheless tolerably slow it opens and I can updates the Ads. But on second server I cannot even go into Ads Places. The Back up server is a copy of primary server.
Suspecting data base I had repaired and optimized no effect. I copy data bases several times no effect too.

Anybody has an idea? I have not tried to downgrade the plugin.

https://wordpress.org/plugins/simple-ads-manager/