OK, so this is for anyone else who might find themselves in the situation I described on my post:

I manage multiple clients with Klaviyo (more than ten, as of last count). One of my client’s accounts seems to be corrupted within Klaviyo, and is causing the behavior that I mentioned in the post, where logging in led to incorrect pages and Klaviyo wouldn’t recognize the content of the account on its pages (no menus, no header, 404 page found, etc.).

Since the client account, for whatever reason, is corrupted, and that was the last account I checked the last time I logged into Klaviyo, the login page led to that account directly on whatever browser or computer you tried (as it normally does), that’s why the normal solution procedures (cache cleanup, etc.) didn’t work.

The only solution for me was to have one of my clients (other than the corrupted account) remove me from their account, then make me a user again. This triggered the ‘accept an invitation’ process to a different, non-corrupted account. I gained access to their Klaviyo account and replaced the corrupted one as the ‘last login’. And now everything works.

I opened two browser windows while I could log in. I switched one of them to the corrupted account and got the same negative result: Klaviyo stopped showing its interface and showed only the conversations/sms page. I closed that tab and went to a ‘healthy’ one, and Klaviyo is working OK there.

As long as I don’t use the corrupt account section, everything is fine, but I don’t know what is causing that behavior or how I can fix it, only that I can get around it because I manage more than one client in Klaviyo.