Please add info about your system, starting with Arastta version, installed template and extensions.

Also some system information: https://arastta.org/docs/user-manual/tools/system-information
Plus your SEO settings: https://arastta.org/docs/user-manual/system/settings/seo

Just tested it in a fresh install of Arastta 1.6.1, where no settings has been changed, works fine there.

Thanks for the quick response.
Even to me it has gone until recently. I do not really use this option and that's why I do not know when it came out.
It's the latest version of Arastta.

The addres where you can see what is happend when compare 2 products is: www.daciaas.ro/hobby2

Platform Arastta 1.6.1 Stable [ Tianjin ] 19-April-2017 15:30 GMT +3
PHP Version 5.5.25
PHP Built On Linux server-0144.whmpanels.com 2.6.32-531.29.2.lve1.3.11.10.el6.x86_64 #1 SMP Fri Jun 12 15:09:02 EDT 2015 x86_64
Database Version 5.1.73-14.12
Database Collation latin1_swedish_ci
Web Server LiteSpeed
Server Interface (SAPI) litespeed
User Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:53.0) Gecko/20100101 Firefox/53.0

At SEO setting everything is ok except Remove index.php witch is YES, because I modified htaccess.txt to .htaccess

Please try to tell us which changes and additions you have made between working and not working, if it did work at the beginning, there surely is something you did.

Also try changing all SEO setting backs to default, then test.
Still issues, then try to also rename the htaccess back to .txt
And/Or eventually take a close look at this part: https://github.com/arastta/arastta/blob/master/htaccess.txt#l23-l27

That's the biggest problem, I do not know what could have happened. From what I remember from the first time I saw the comparison between products, I did nothing but put filters and attributes on products, which is very difficult, by the way. I will try to do all the operations you have indicated to me and I will come back with information.
Thank you very much

Surely I did something, but because I am inexperienced guy, I thought you might have come across it and it's easier to find and fix the problem.

1. Take notes of system changes and additions
2. Provide needed info
3. Test and check everything you're asked to
4. Do your homework

5. Don't relay on people having a crystal ball.