That is one of the fixes on last FW
Fix a bug that TL-WA7510N cannot connect to root ap under client/repeater mode if the encryption is WPA/WPA2-PSK.
You can always flash and test DD-WRT

Going to give DD-WRT a go.

Set it to WPA-PSK (not 2), AES and it has been working for a few hours without a problem. Did try dd-wrt but reflashed back to TP-Link as I didn't have time to workout the setup. Didn't get as far as security. Also found dd-wrt has a very slow interface.

good to hear
post after a week to see if that solves your problem.
about

Did try dd-wrt but reflashed back to TP-Link as I didn't have time to workout the setup. Didn't get as far as security

login root older versions leave login blank
password admin
http://www.dd-wrt.com/wiki/index.php/Reset_And_Reboot

12 to 14 hours later and the problem is back. The client sees the AP. I can do a signal test. I can join the AP. But nothing passes through.

I now only run MAC filtering on the AP. So only my client can connect. Not the best security, but it all there is.

1) Insure the two 7510 use the same Region
2) Disable DFS.
Or
change the Region to Test Mode

Did you ever get this working - I've got two of these and they only seem to work using no security or WEP, which is completely useless, and from the sounds of it stops working after a period. Like you, I've had to resort to MAC filtering which as far as I'm concerned is unfit for purpose.

Guys, I have two WA7510N running between my place and my brother's place. I have try different modes(AP/Client/Bridge) after I purchased them. This problem never occurs at my side.

I've set the two 7510 as below:
1) the same "Region";
2) one as Access Point, the other as Client;
3) disable "DFS";(if available)
4) encryption WPA-PSK/WPA2-PSK;
5) other Default

I wonder how you configure and place your devices? Can the problem due to the "Group Key = 30"?

Tobacco Congratulations you have nailed this post, personally I didn't know about Group Key Update Period by default most TL devices is 0 means no update, 5110 and 5210 default is 86400
DairyFarmer enable it or his 7510s came to minimum 30 seconds, interval where the encryption key is automatically changed, reason why when disable security everything works

use WPA-PSK and put the group key update period to " 0 "

also in system tools enable ping watchdog in each device ..