Can't share a vault with a Guest

Hi everyone,

I'm hoping someone in the community has run into this before, because I've exhausted what I can try on my side and I'm still waiting on a reply from Support and Success.

The situation:

I invited a client to my Business account as a Guest. He accepted the invitation, created his account, and I confirmed his access. His status is now Active and he is not assigned to any vault. However, every time I try to add him to a vault, I get this error:

Failed to add person to vault. An unexpected error occurred.

Error Code: 400

What I've already checked / tried:

• I'm the only team member on the account and I hold owner/administrator permissions, so I have full rights to manage vaults and people.
• The guest is not assigned to any other vault.
• Tried adding him from the vault side (Manage Access) and from the person side (People → user profile → Vaults). Both fail with the same error.
• Deleted the user entirely and re-invited him from scratch. He accepted again, I confirmed him again, and the error persists.
• Tried a different browser and an incognito window — same result.

Has anyone seen this before? Is there something I might be missing, or a workaround that's worked for you?

Thanks in advance for any pointers.