Use the current domain if the api path is relative to the frontend host

src/components/misc/api-config.vue

@@ -54,6 +54,9 @@ export default {
 	},
 	methods: {
 		apiDomain() {
+			if (window.API_URL.startsWith('/api/v1')) {
+				return window.location.host
+			}
 			const urlParts = window.API_URL.replace('http://', '').replace('https://', '').split(/[/?#]/)
 			return urlParts[0]
 		},
@@ -158,6 +161,7 @@ export default {
 						this.successMsg = `Using Vikunja installation at "${this.apiDomain()}".`
 						localStorage.setItem('API_URL', window.API_URL)
 						this.configureApi = false
+						this.apiUrl = window.API_URL
 					}
 				})
 		},