A complete rewrite of the COL Reminder Web is just gone live.
This fixes the latest problems with not getting notifications on the WEB and also it was not possible to add new reminders from the WEB to the phone.
More details why this took so long here:
https://shlink.christiangrasser.at/colreminderupdate
You need to update the COL Reminder Android app to 3.7.6.4 to get this full working.
COL Reminder Web:
https://web.colreminder.com
Play Store:
https://play.google.com/store/apps/details?id=com.colapps.reminder