I've already solved it, what a madness. It turns out that I have a landing page where you log in to an intranet(opener) that opens a book(opened) and after a few seconds both the opener and the opened just froze, if you copy and paste their urls in a new tab or I reloaded the opener, everything worked perfectly until the session was started again, which froze again. Comment that in the book, Animate (createjs) is used a lot and it is possible that it is a matter of the conjunction of both products. I had another landing from which I could login and none of this happened, and it turned out that in the landing that freezes I had a YouTube video without a posterframe, so I guess it preloads the video (I see a loop of calls to youtube, without touching the video) and this is what then freezes both the intranet (which is loaded where the landing page was before) and the open book from the intranet. It was enough for me to put a posterframe on the landing and everything works as it should. Thank you all for the invaluable help, I have not seen any error log anywhere that could make me think of this big mess because of a video that is on a previous page. Of course this does not happen in versions of iOS prior to 15.4 (or Safari prior to 15.4), and I still can't explain how it can happen, at least it will serve as a clue for someone.

Just to expand, I just verified that on my MacBook Air with Safari 15.4 it just started with the same symptoms. In this case, only in Safari, Chrome works fine.

To enlarge, once both pages are frozen, the opener looks gray, but if I reload it (after several attempts to do so) when it finally reloads, the opened window is also automatically reloaded and it doesn't crash again until I start the session again.

I have found these errors in the console of the ipad from the mac, but I do not find experiences or explanations about it. error 13:27:19.866261+0200 SpringBoard Didn't find grabberView for error 13:27:19.907176+0200 SpringBoard Didn't find grabberView for error 13:27:19.916574+0200 sharingd Trying to grab power assertion while we already have one error 13:27:19.919697+0200 MobileSafari Detected stale pending counter; resetting to 0 error 13:27:19.929800+0200 ContextService Safari request with no text error 13:27:20.241384+0200 ContextService Safari request with no text error 13:27:21.009792+0200 dprivacyd : Disabled by D&U switch error 13:27:21.009974+0200 sharingd Trying to grab power assertion while we already have one error 13:27:21.095180+0200 ContextService Safari request with no text error 13:27:21.245225+0200 kernel Failed to read key 1195856451 from SMC with error code 84 error 13:27:21.290565+0200 kernel Sandbox: hook..execve() killing com.apple.adattributiond[pid=19781, uid=0]: (err=22) failed to assign builtin profile error 13:27:24.103899+0200 bluetoothd stopTrackingForSession: sensor tracking is not supported on this platform error 13:27:24.108433+0200 bluetoothd stopTrackingForSession: sensor tracking is not supported on this platform error 13:27:25.250444+0200 kernel Failed to read key 1195856451 from SMC with error code 84 error 13:27:26.587202+0200 SpringBoard Error acquiring assertion: <Error Domain=RBSAssertionErrorDomain Code=2 "Assertion's invalidation time is in the past" UserInfo={IgnoreOnReconnect=true, NSLocalizedFailureReason=Assertion's invalidation time is in the past}> error 13:27:26.587281+0200 SpringBoard [<_UIKeyboardArbiterHandle: 0x283ff4210; PID 2915: com.apple.AuthKitUIService <(null)>; hosting PIDs {( )}; level 0.000000; active NO [wants NO]; suppression 0; iav 0.000000; on screen NO>] could not take process assertion error 13:27:27.581008+0200 MobileSafari Touch event gesture recognizer failed to reset after ending gesture deferral: error 13:27:29.256721+0200 kernel Failed to read key 1195856451 from SMC with error code 84 error 13:27:31.019996+0200 MobileSafari sendUserActivityToServer, called on activity 2B5FF40F-3FDE-4B75-8B94-6263C7590F5C after it had been invalidated, so doing nothing. error 13:27:31.319005+0200 kernel Sandbox: hook..execve() killing com.apple.adattributiond[pid=19782, uid=0]: (err=22) failed to assign builtin profile error 13:27:33.262729+0200 kernel Failed to read key 1195856451 from SMC with error code 84 error 13:27:34.136189+0200 bluetoothd stopTrackingForSession: sensor tracking is not supported on this platform error 13:27:34.141164+0200 bluetoothd stopTrackingForSession: sensor tracking is not supported on this platform error 13:27:34.616755+0200 sharingd Trying to grab power assertion while we already have one error 13:27:35.112416+0200 wifid WiFiDeviceSetProperty error 13:27:37.268742+0200 kernel Failed to read key 1195856451 from SMC with error code 84