Endless new plugin search uninstall error

Three weeks ago my ISP informed me that my site was suspended as it was sending spam.

The ISP gave me a list of infect files its antivirus found among my files.

I cleaned all that files and then it was back online.

I installed an antivirus plugin.

Everything seemed to work: I though to a happy ending.

But since then the search for a new plugin starts in endless loop, and the removing plugin function yields an error.

I googled and I found a post of a guy having a similar problem, his solution was to change the access properties of the folder wp-content/update. It didn’t worked for me.

Have you any other hint?

Thanks in advance, Federico