Changes have to be made to be compatible with Bootstrap 5. Here an example where I have updated the popup.qgs project: https://github.com/3liz/lizmap-web-client/pull/4455/commits/77dbff73237f74d30d0ad7478913049f896e2851#diff-946651685ba8d94c9f080650b7b7eac0788927468349392f0ad624fcee3a24f3R1044-R1046 We also have to think about legacy popups compatible w/ Bootstrap 2.