[Koha] Hold problems

Dear all,  I need a help.  I have problems with Koha's Hold feature. Patron can place a hold but cannot cancel it. For cancelling a hold the message is: "Sorry, the requested page is not available. Error 500" Staff can check in a hold, bu cannot: - check out a hold - suspend a hold - delete a hold For these the error message is: "An error has occurred! Error 500  How to solve this problem? Any clue and help would be greatly appreciated. I install the latest Koha (21.05) in docker. Thank you. Best regards, K. Gunawan