Please tell me what are the requirements for a password in WinBox (number of letters, digits, etc.)?
MikroTik hAP ac2, RouterOS 6.44.6, WinBox 3.24.
None as Winbox is just a tool passing passwords to devices to login.
ОК, MikroTik hAP ac2 or RouterOS 6.44.6?
No difference … there is no build-in rule for passwords limits
IIt is not true. The password can be up to ~65K characters long… 
When you made an error in a 43765 letters long password
OK, thanks!
Is the password case sensitive (upper and lower case)?
Yes. I suggest to stay with ASCII set of characters. No experience with UTF or other “strange” combinations.