Plugin doesn't work. 403 forbidden error instead

Hello,

for the past 5-6 days the free version of the plugin stopped working and instead it outputs a 403 forbidden page and does not login ANY user (even the administrator).

I placed the widget in various places but it had the same behaviour. I updated in the newest version and returned all options to the most basic of forms, but to no avail.

For your convenience I provide you the link and a simple user name and password so you can try it out:

http://tinyurl.com/zjvbk2u

username : TestSubscriber
password : 123456

Thank you for your time and your plugin (so far it did a perfect job !).

P.s.
(in case tinyurl link doesn’t work, this is the link : http://www.palmosev.gr/palmos/%CE%B7%CE%BB%CE%B5%CE%BA%CF%84%CF%81%CE%BF%CE%BD%CE%B9%CE%BA%CF%8C-%CF%84%CE%B5%CF%8D%CF%87%CE%BF%CF%82)
it contains greek characters that is why it looks like that.
I tried placing the plugin in a page with standard english characters, still got the same problem.

p.s.2
I forgot to add that I tried the compatibility option. No luck. Currently the option is disabled so that no other addon intereferes with it.

https://ww.wp.xz.cn/plugins/login-sidebar-widget/