Strange error with Outlook 2010 : 30 time today on the server, about 4 or 5 just for me...
 
SM17 (100.0.6996)

It's a POP account.

[2019.03.06] 18:51:07.308 [XX.XX.XX.XX][6841779] connected at 06/03/2019 18:51:07
[2019.03.06] 18:51:07.339 [XX.XX.XX.XX][6841779] CAPA
[2019.03.06] 18:51:07.355 [XX.XX.XX.XX][6841779] USER email@address.net
[2019.03.06] 18:51:07.386 [XX.XX.XX.XX][6841779] PASS XXXX
[2019.03.06] 18:51:07.448 [XX.XX.XX.XX][6841779]Exception: Value cannot be null.
[2019.03.06]    at System.Threading.Monitor.ReliableEnterTimeout(Object obj, Int32 timeout, Boolean& lockTaken)   at MailService.Mailboxes.Mailbox.FreeMemory()   at MailService.Mailboxes.MailboxManager.FreeMemory()   at MailService.Core.Mailman.CleanCacheIfRequired()   at MailService.Core.Mailman.GetMailboxManager(String logicalAddress, MailboxManager& mm)   at MailService.Core.Mailman.RequestMailbox(String logicalAddress, String sessionID, MailboxManager& mailboxManager)   at MailService.TcpServerLib.POP.POPSession.ValidateUser(String userName, String password, Boolean& isLoginCached)   at MailService.TcpServerLib.POP.POPSession.PASS(String argsText)   at MailService.TcpServerLib.POP.POPSession.ProcessAsyncCommand(String popCommandText)18:51:07.480 [XX.XX.XX.XX][6841779] disconnected at 06/03/2019 18:51:07

Outlook asks for the password...I just clic [ OK ] and everything is "ok"