I havent faced any problem with online report. This is the link you'll be needing:
https://tm47.immigration.go.th/tm47/#/login

If you've never registered before, you need to do that first here:
https://tm47.immigration.go.th/tm47/#/register/add

If it's the first time you've ever filed a 90-day report or you've just changed your passport, you'll need to do it in person at Thai Immigration. After that, it can be done online.

The online 90-day reporting has always been a bit hit-and-miss. Sometimes it works, sometimes it doesn't.

I've heard of people who’d been doing their 90 day reports online for years without any issues, only to suddenly get rejected by the system (for reasons they couldn't understand).. That was enough of a red flag for me! So I do all my reports in person now.

Thanks guys!