Messages crashes at launch

Question

Messages app keeps crashing at launch, reporting of an "internal error in Messages" and forcing me to quit it. This sometimes leads to loosing newer messages; they disappear from the feed altogether.
It started to happen on my system when Messages got this feature that it launches by itself when a new message has arrived while the user was offline. But it happens also when Messages launches automatically at login (set in Preferences).
I would rather launch Messages always manually, but it is not possible because of that feature.  It never asks anything (quitting is the only option) and I haven't found a setting to disable the feature. If there is any way to disable it, I would be very happy to hear!
Why the "internal error" appears, I have only one clue. It might be related to backup. I use iDrive backup which also launches automatically if it has missed its scheduled backup. When iDrive's automatic backup and Messages' automatic launch (particularly the "by itself feature") coincide, Messages crashes. However, this is not totally consistent and hence I can't say anything for sure.
If anyone can shed any light on this - or offer a Terminal command that disables the unwanted "by itself feature", I would much appreciate! The loosing of new messages is pretty serious.

Answer 1

Please read this whole message before doing anything.
Back up all data.
Quit Messages if it’s running.
Step 1
Hold down the option key and select
Go ▹ Library
from the Finder menu bar. Move the following items from the Library folder to the Trash (either may not exist):
Caches/com.apple.Messages
Caches/com.apple.imfoundation.IMRemoteURLConnectionAgent
Leave the Library folder open. Log out and log back in. Try Messages again. If it works now, stop here. Close the Library folder.
Step 2
If you still have problems, quit Messages again. Go back to the Finder and move the following item from the open Library folder to the Desktop:
Messages
Note: you are not moving the Messages application. You’re moving a folder named “Messages.”
Test. If Messages now works, delete the Messages folder on the Desktop. Otherwise, quit Messages again. Put back the folder you moved, overwriting the newer one that may have been created in its place, and continue.
Step 3
In the Preferences subfolder, there may be several files having names that begin with any of the following strings:
com.apple.iChat
com.apple.imdsmsrecordstore
com.apple.imessage
com.apple.imservice
Move them all to the Desktop. There may also be a file with the name "com.apple.imagent.plist". Move that to the Trash.
Also in the Preferences folder, there's a subfolder named "ByHost". Open it and do the same thing.
Log out and log back in. Test again. This time Messages should perform normally, but your settings will be lost. You may be able to put back some of the files you moved to the Desktop in this step. Relaunch and test after each one. Eventually you should find one or more that causes Messages to malfunction. Delete those files and recreate whatever settings they contained.
If the issue is still not resolved, quit Messages again and put all the items you moved to the Desktop back where they were. You don’t need to replace the items you moved to the Trash. Stop here and post again.
If you later decide that you don’t like the results of steps 2 and 3, you can undo them completely by quitting Messages and restoring the items you deleted in those steps from your backup.