1. You have already downloaded the flash2.hex inside 8252 and now you can't overwrite with the new one ?.
2. At which COM port is attached your mouse ?
What kind of downloader you use and what programming mode : serial or parallel. Which PC device the data comes from: COM port or parallel port ?
Are you sure that write lock bits haven't been enabled at the time when you downloaded first time the flash2.hex ?
And if they have been enabled, are you sure that the first operation which start the programming process is Chip Erase ?
Since you use serial downloading method are you sure you follow the serial programming altgorithm in the right sequence ?

You wrote:
If i make a reset of the 8252 the PC works

Maybe this is the main reason for which you can't program the chip:
As you know, the first step in serial programming altgoritm is setting RST pin to high. (Now I'm wonder how you set this by hand, what the logic level you apply to RST pin and how is wired the PC command downloader to your target device)
Your mistake is that you use P1 port as flashing your bits in application. The downloader use it too. P1.5 like MOSI, p1.7 like SCK.
Once again, what kind of downloader you use ? Maybe your downloader is unable to raise the RST pin to high and the 8052 is running your application as time you try to download. If you use the serial programming mode the conflict is obvious.

If nothing works, try to program the device in parallel programming mode.
Change in flash2.asm the port P1 with P2, compile, download serial the program and see what happens. Just for test.

For future:
Try to jump across vector table. You can do this by immediate place a JMP main, after ORG 0000H. And write immediate before the main label, ORG XXXXH where XXXX is greater than 0050H (for future updates)
Place colon immediate after every label.

Regards,
Silviu