Q: iMessage "An error occurred during activation. Try again."

Apple's servers are overloaded with iOS 7 downloads and activation requests.

Be patient and try again later.

Cheers :D

Back up your phone to icloud or your computer.  Then go to Settings > General > Reset > Erase all content and Settings. 

After the phone is erased, you can restore all of the information from icloud or your computer.  imessage works now.   This worked for me!

I tried many things from other posts.

What worked for me.

I made sure that my number (under phone) was complete with the country area code. (As this is what I had for my Apple ID)

After I did this and reset my phone (holding both top and bottom button) i signed in again and it worked.

Same problem on ipad mini wifi model. Can't do any of the suggested fixers either because in the settings menu I cannot even turn off iMessage/FaceTime since I can't sign in.

Absolutely embarrassing to say the least that this hasn't been fixed yet!!

I agree with MrZboii, tried all the steps and still is not working.  Upgraded to the IOS7.3.1, supposed to fix the problem, but it has not.  Extremely frustrating that Apple has not fixed this yet!

What Apple think about this problem. I can't activate imessage. I have better things to do then reseting, activating and deactivating settings. Is there any solutions for iPad without 3G.

I had iMessage "An errror occured during activation..." On my iPad mini retina running iOS7.0.4 while trying to sync iMessages w/my iPhone 5c iOS 7.0.4. I followed an example someone posted on this thread. Back up iPad via iTunes on my mac, then on iPad Settings/General/Erase All Content and Settings. I did notice after restoring my iPad via iTunes that my iPad's FaceTime had a popup box asking for some permission...I believe it may have been "FaceTime" that I had synced with my iPhone which could've been the problem from the start? But either way, my iPad iMessages syncs w/my iPhone now.

After hours of battling and trying numerous things - I found that by signing out of Find Friends resolved the iMessage issue and allowed me to sign in again - this is on iPhone4S with 7.0.4. 

There was a little sign out button on the app where it asks for a password.

Interestingly, if I wiped the phone and started 'vanilla', then I could sign in.  Restoring my backup resulted in me no longer can sign in, until I signed out of Find Friends.

Also a few days ago Apple made me change my AppleID password - this might have been the start of a downward spiral of stuff not working.

madmusic since you have a still in process ios version on your phone of course things on here will not fix it.

I experienced this issue after restoring a backup from an old iPhone to a new iPhone 5c. I restored to factory settings, then tried the restore from backup again, which worked. You can follow this blog post for instructions.

Great detailed explanation but unfortunately it is not working on my Iphone 5C iOs 7.

Any other recommendation?

Thanks

A huge thank you to 'SissyRibbons'!

I recently reset my entire iPad at the beginning of the year, for nearly two months I had no clue how to get my e-mails, iMessage or FaceTime working due to my the 'iMessage activation error' - my Father didn't even know what to do and frankly, the 'apple support communities' were no help to me at all! Until I came across this post!

Big, big thank you to Teeneylyn for posting on behalf of SissyRibbons!

After trying just about everything, including SissyRibbons suggested ChaCha and Hokey Pokey, I realized under
Settings > Phone > My Number I didn't have the 1 before my area code. 
If you are in the U.S.A. it needs to say 1(xxx) xxx-xxxx.  Took several hours for Apple to verify my new number now it works like a charm.

Hopes this helps!

go find sissyribbons in this very long stream of answers."
go to her. she will tell you your destiny.

no seriously. she has the answer. worked for me when i accidentally turned off my iMessaging and then it no longer worked.

GO! FIND SISSYRIBBONS! FIND HER (him?)!!!!

I actually just reset and did not erase content and it fixed it as well.  Thanks!