Either there is some kind of snafu that has caused corruption of the master password hash that your browser extension uses when verifying sensitive actions (master password reprompts or vaults exports), or your browser extension is in fact logged in to a different account, which has a different master password than your “true” account.
I would suggest that you start by going to the Account Actions page in your browser extension, and examine the email address and server domain displayed there (see instructions here). Carefully compare these pieces of information against the base domain of the website that you use to access the Web Vault (i.e., bitwarden.com vs. bitwarden.eu), and the email address displayed under Settings > My Account in the Web Vault app.
If the email address and server domain match exactly, and if you have no other reason to believe that the vault data available in your browser extension is in any way different from the vault data that you access in the Web Vault, then I would suggest that you log out completely from the browser extension (by accessing Account Actions via the profile avatar icon, and selecting 
Log Out. Make sure that the browser extension icon (next to your browser’s address bar) turns gray.
If you specify the same server domain and email address that you use to access the Web Vault, then you should be able to log back in to the browser extension. Hopefully this clears out any corrupt hash values, and you will now be able to use your master password to access items protected by master password reprompt, and to create exports.