Message not sent. Click to retry.

Hello,

I started using the web version of Google SMS to manage my texts more easily, but my failure rate is very concerning. Sometimes, the numbers do not receive my first message (so they may have been deactivated in the meantime), but the conversation has been initiated with others before suddenly stopping with the error message "Message not sent. Click to retry" for the web version and "Not sent. See options" for mobile.
I tested with two other numbers I control for a result that doesn't help me much. It works for one but not the other (in SMS, as I can call both).
I can't narrow down the problem and therefore find a solution. I have already tried the basic stuff (clearing cache, forcing stop, removing and re-adding RCS, etc.).
Do you have any ideas to resolve this? Thank you!
To summarize the situation:
- Unable to send to certain numbers
- Others stop working during several conversations
- It's not a timing issue (failure then success in a row with my two numbers)
- Same situation whether on web or mobile