Why cannot open any excel files after enable "Ignore other application that use Dyanamic Data Exchange (DDE)"

It is not totally clear which problem you try to solve by switching off DDE. DDE is used only for finding and controlling running instances.
You should always be able to start Excel manually in multiple instances.
It should always work to open sheets if done in Excel itself.

So I guess your issue is only with clicking on files in Explorer?

Thank you so much for your reply. My final purpose is to open multiple excel files in multiple instances. we already tried many ways to do that, such as, open anther excel instance first and then open another excel file from the new instance. and also, we can modify registry of PC to resolve the issue etc. but enable  "Ignore other application that use Dyanamic Data Exchange (DDE)" is best way. however, this way only can work for several days in my PC and then i have to disable it because i suddenly cannot open all excel files unless disable  "Ignore other application that use Dyanamic Data Exchange (DDE)"

Your link discussed a lot on the issue. It seems changing registry is a good way. Some one changed registry for the issue successfully. but later the issue come back again. they explained the issue coming back is due to installation of new software. Do you think so? If so, how can i prevent it from happening again if another software need to install in future? In fact, my pc registry changing many times for this issue, so it caused the issue goes back again again.

Do you think so? If so, how can i prevent it from happening again if another software need to install in future?

I cannot answer either of these questions. Instead, I suggest you conduct an experiment to see if the suggested registry tweak fixes your problem.

Why cannot open any excel files after enable "Ignore other application that use Dyanamic Data Exchange (DDE)"

Because you need to switch off the usage of DDE in the registry, too!

In Excel2010, current best way is to change registry. I did it, but issue is after updating some software, the registry changed back. So I hope there is a way which can fix the changed registry and keep the registry not changed back.