Created attachment 48220 [details] Screen shots of new subscription form entries, and software errors we got after Serials crashes When adding a new subscription, and after filling all fields on form, reviewing patterns, click on "Save", Serials crashes and gives the attached message.
It crashes because of the invalid subscription end date 9999. It does not crash with current version 3.22 and with current master. Workaround for 3.18: Leave empty or enter a valid date for end date. BTW: It seems that the end date should be empty while the subscription is running, see Bug 7728 More related bugs: Bug 15484, Bug 15149
Version 3.18.x is no longer maintained, so there is no path forward for a fix for this. The best solution is either to use the workaround or to upgrade Koha.