Is it okay to attend the lectures/classes of the Al Kauthar Institute?
The Al Kauthar institute is like the al-Maghrib institute; they do not ascribe to Salafiyyah nor invite the people to the Scholars. They have speakers who have also made major errors in dawah like aligning themselves with the people of innovation. So, I would advise avoiding them and their courses and instead take knowledge from those whom are known to be upon the methodology of the Salaf us-Salih. As Ibn Sireen rahimahullah said:
إن هذا العلم دين فانظروا عمن تأخذون دينكم
Verily this knowledge is religion so look to whom you take your religion from
[Saheeh Muslim in his introduction with a Saheeh chain]
And Allah knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan