Cookie Monster

First of all the title is a bit misleading, everyone needs it "immediately" so no need to emphasise it.
Now there are a few things when you change the domain.
1) Cloudflare cache (No idea check with them even disable it while it's aggregating and getting new domain info)
2) AdminCP > Support > Something is not working will clear the cache on your forums.

I have no clue what you two are on about he asked for help. So you guys just replying without reading and needing to post @ChrizyS @Maquiavel
@Xerin I would suggest to hire a developer to do this for you if your not comfortable with HTML / CSS / JS as that is all it takes to create it.

You need to allow the person or group that can post it to post it as HTML and ahve access to write HTML. Click on SOURCE then paste it to redo it

Could just use the Topic Feed block and select it to only show if featured then you "feature" tag all topics that needs to be an announcement.

https://invisioncommunity.com/services/migrate/
Select MyBB and see what it can migrate by using the converter that's built in.
Though would suggest 4.3

Restore a backup and reconvert it then

For other people.
You can not have PHP version 7.3 installed or IPB/IPS will not work. 7.2 works.

You database will start to grow, the only other thing that will grow would be when people upload images etc to the forum.
Why don't keep an eye on it and if it grows and it has a gig or something left you buy more space or find a new host to move to ?

That is a backup file temp named by the system and is is not completed.

make sure you got the right php version installed. If this is a clean install try reupload the files again to make sure nothing is corrupt.
 
Have you installed any plugins or applications ?

I think you can hide it with CSS simply and then add just upload and attach it to the usergroup.
Should technically be a 1 line CSS thing to hide the text under if that's what you want.

Well the plugin you installed is for 4.1 4.2+ I believe is when reactions came out? So it's not compatible would be my guess.

AdminCP > System > Support > Something is not working.
 

To me, that looks correct.
Your Forums with announcements is holding Welcome! and you have a sub-forum inside of it named "Forum Support" which is a subforum of the normal one.

I would just write a tiny bit of jquery to make this work.
$(document).ready(function() { $("a").live("click", function() { if (this.href.indexOf("yourwebsite.com") == -1) { window.open(this.href, "target=_blank"); return false; } }); }); replace ofc yourwebsite.com and also put it in custom.js or create it.
This will add all A links and attach target blank to them IF they ar enot part of yourdomain.com

AdminCP > System > Site Features > Menu Manager
Reorder so its not a child of Browse.

You can create the block yourself, and copy the code from Post Feed Block
In IPS Pages you can create a block and use Post Feed Block as a template, then you edit the loop to only look for the first one or only display it like that.
If you do not know how to code, you can use the marketplace here to hire a developer for you to do it.

Admincp > system > support > soemthing is not working. 

Well, that or try upgrade to the last one, but yes that's the one causing the issue.

4.2 you can set a group highlight colour in group settings.

Backup files and database 
Overwrite all files with new files
Go to /admin/upgrade and follow on screen instructions

Admincp > search engine optimization > meta tags 

They linking a movie like youtube or Uploading it like an attachment.
If it's an attachment it's a few things
403 errors mean "no permissions / no access" this can then be a permission issue with uploads folder etc. Check that its CHOMOD 755 for the folder and subfolders etc.
Can it also be the amount of upload they are allowed? Whats the max_post that you have set to allow?

So try edit your php.ini file with something like this
upload_max_filesize = 128M post_max_size = 128M memory_limit = 300M file_uploads = On max_execution_time = 180 Bit excessive but should work

Well your saying deny everything except if it's from up 69.69.69.69
So the cdn will not work
So simple delete your htaccess and rebuild it in admincp > system > search engine optimisation