No access to Slack when connected to SSLVPN

Hi,

We currently have some weird issues when connected to SSLVPN.

We have split tunneling configured already so the traffic to the internet is separated of the VPN. But for some reason, some users when working from home they lose access to some websites like Slack. The application and the website stops working. We contacted slack and they said we should contact the ISP of the users for that. But I'm wondering if there's any configuration I can do to fix it without routing all the traffic over the VPN. Also, I haven't come across yet with ISP that blocks traffic from SSLVPN.

If it's worth mention, those users complaining are based in Romania.

Thanks