I believe this is the same issue that was brought up a while ago in this topic.
I have been experiencing the same thing since that topic was brought up nearly a year ago, but only on one of the two computers on which I use Manager. In that time I have updated Windows and Manager several times. I am now on Desktop version 21.10.6 of Manager and Windows 10 version 20H2.
I was about to post to the above topic to see if anyone else is still experiencing the issue, as this morning it took me about four attempts and 20 minutes to open Manager. Each time it gives the error it appears to be loading for a while before the message appears, which is why it takes so long.