First of all: structural rebuild does not harm user databases. It will create a new user database out of the old one WITHOUT deleting folders, settings, ...

A "Rebuild user database" will do the activities you fear.

Therefore a structural rebuild will help and does not hurt. A "Recreate user database" is dangerous and the last way to fix issues.

Nevertheless I had a similar customer request yesterday. Client hangs after login (not because of calenders). I did some gwchecks but nothing helped. Finally a poa restart let shine the sun again ... 

I tried structural rebuild and GW restart.
First it seems to work, but after a short time the problems appeared again.

I need some additional information. Background system is on version 18.3 too - that's the reason why you use client version 18.3. Right?

Can you tell us your build number? I know there are some patches available.

Are you allowed to open a case (and did you)?

Maybe I have some thoughts around multiuser calendars but I will return to this later ...

If that happens for more users and on more appointments, then this can be a code problem but for such an old GW version, there will not be any bug fixes anymore. You will need to upgrade.

If it is only a few appointments, then perhaps try to archive it .. and then unarchive and see if that helps with editing this item.

Client version: 18.3.1 15.05.2021

Build 138096

It's only one user, but he works excessive with multiple calendars ...

If this problem is linked to his workstation, that you might try letting this user work on someone else's PC for a day and see if there is no client crash. Check on his current PC network settings -> see if his network is set to Public or Private and if it is Public, delete it and create a Private one. I would also disable IPv6 protocol on his PC.

That is a version I can offer: 697364-gw18.3.1-140372-194_client_win_multi.exe

Multi-User calendars or multiple calendars?

I have seen issues (not only GW18.3)  if there are a lot of multiuser calendars in the calendar bar

You do not have these issues if you have a lot of multiuser calendars on the left side (folder view); of course do not enable them all together because that means a lot of fetching activities for the client.