MacBook Pro startup screen freezes

I have run EtreCheck and followed its recommendations. Reboot, and *boom* it appears that the problem has been solved. I shut down and rebooted again, just to be sure it wasn't a vicious tease. It works. USB connections work, quickly checked some of my pet code, and all seems to be well. I made a screenshot of the items removed, but I have no idea which one was the culprit. I hope that I can attach it herewith.

I will buy the paid version of EtreCheck, first as a thank-you, second to delve a bit deeper just for good measure.

As to a thank-you for my distant rescuer: if ever you're in Leiden, I'll buy you a good lunch, whatever your diet persuasion may be (this is a student town).

Thank you, I'll check the link you sent.

Meanwhile: what I see is the usual Catalina startup screen, with the rocky island in the ocean, and perfectly normal rectangle for my password. The mouse pointer is top left screen as usual. At that moment, the mouse responds and I can start typing my password. But five or six seconds after startup the whole thing freezes (i.e. no response to mouse, trackpad or keyboard), before I have even had time to complete my password. I can get out only by using the 6-second push on the start button. I have even tried to let it run all night, to check whether here was something unusually slow in the startup chain, but that did not help.

Further to my reply (below):

Hi brbo, thanks for the link. I had in fact seen those images before somewhere, but my problem isn't among them. I forgot to mention that I have run all the usual available tests of my system, but none of these reports anything amiss. No probs with the SSD or other hardware. I have no idea whether it is possible for me to check the firmware. I am aware that my stuff is old (by today's standards), and that maybe I should upgrade the macOS beyond Catalina. Problem with that is, that some of the non-Apple software that I use (mostly stuff for my science research) might break. Plus, of course, that it would not answer the question: what's wrong?

Thanks once again! Got 122 Gb free, should be enough I hope. Only antivirus is Malwarebytes Pro, temporarily disabled: doesn't solve the prob. No cleaners. Adblocker only in Firefox, not running during startup.

I'll check your Catalina and Etrecheck (does that run in Safe Mode?)

Thanks! - Vincent

I have now run EtreCheck (thanks for the hint). The report appears to be quite complete. I have removed the fossils it found. Also read the list of [Not Loaded] extensions is provided, because these probably didn't load because of Safe Mode. The report doesn't seem to be interesting enough to share at the moment.

I've used the "Ways to start..." page before. Verbose mode turns out to be way above my comprehension, the rest I've used (but no paths to solution).

I'll reboot later in ordinary mode to see what difference the EtreCheck recommendations made.

Good news it is, thanks to you & EtreCheck. Australia I know only from visits to Mt Stromlo Observatory, for various conferences. Leiden is almost diametrically opposed, on Planet Earth. Cheers, & thanks again!