Looking through my logs I found at least one of those log entrys for each user trying to/actually authenticating.
The error seems to appear even if the login was successful, but I'm investigating a bunch of reports, where users where not able to login after all. Looking through my google results nothing came up, which would have helped. Checked all the fba settings, but since I can't reproduce the failed login, the error message is everything I can stick to at the moment.
Event code: 4006 Event message: Membership credential verification failed. Event time: 7/8/2012 12:53:37 AM Event time (UTC): 7/7/2012 10:53:37 PM Event ID: c97aa3622d30422e8552883c4292361f Event sequence: 2416 Event occurrence: 11 Event detail code: 0 Application information: Application domain: /LM/W3SVC/1867074284/ROOT-1-129860944820252444 Trust level: WSS_Minimal Application Virtual Path: / Application Path: C:\inetpub\wwwroot\wss\VirtualDirectories\80\ Machine name: **** Process information: Process ID: 3388 Process name: w3wp.exe Account name: ****\svc_sp_**** Request information: Request URL: https://****/webpartpages/accountrequest.aspx Request path: /webpartpages/accountrequest.aspx User host address: **** User: Is authenticated: False Authentication Type: Thread account name: ****\**** Name to authenticate: usernametoauthenticate Custom event details: