Auto-increment field stopped working at 10000, any suggestions very welcome

I have added an auto-increment field to the accounts module. I have set it up as follows:
The format field is {ACCT}{-}{}{}{0000000}{}{}

It incremented perfectly for many months until it reached 10000. After that all the records created after these dates have the same value, ie ACCT-0010000

I have another module, the contracts module, to which I have added a similar field and the exact same thing has happened, on a different day.

I am baffled, does anybody have any idea what I am doing wrong?

Thank you very much for any advice.

Which version you are at ? and which field type you are using?

Hello Ashish and thank you for your interest. Here is the full field definition:

The version num is as follows (taken from the About window):

Version 7.10.27
Sugar Version 6.5.25 (Build 344)