I haven't seen the file attached, but you can try with the Accessibility tool and run the "Accessibility Check".
It will produce a report with insightful information to point you in the right direction of what else could be wrong.
My question is, are you not able to clear the digital signature?
And when you try to clear it, is that the error message that you get?
I've been experiencing this issue myself with my PDFs.
It seems like a bug that corrupts the digital signature.
Specifically, I've observed this happen when, IF during signing time, you select an signing identity with a trusted certificate that requires to be online to get a time-stamp from the issuing certificate authority.
For example, as you click on the " Sign" button, it will prompt you to "Save As".
However, if just before you save the file you click on Cancel, you'll get a pop up message saying " The document could not be signed".
Acrobat will still go ahead and execute the signing action bit it will not succeed in validating nor finalizing the signing action.
The weird part is that while the document still gets signed regardless of the cancelled action, it will not be "Saved As under a different file name. Moreover, in some instances it will hang or stall Acrobat forcing you to restart the whole program again.
Next time that you restart Acrobat and recover the document, it will appear as certified with a signature plus you'll not be able to remove such signature because is now corrupted.
In other words, If you didn't save a copy of the original work before it was signed, you may as well say "good bye" to all the content you were trying to edit... it won't let you.
Is this is what happened to you?
Your signed document is broken in a myriad ways.
In particular the ByteRange value indicating starts and ends of the signed byte ranges does not match at all the PDF structure.
A possible cause for this is that there was a properly signed PDF in the beginning which then was processed by some software that did not save its changes in an appended incremental update (which keeps the cryptographic validity of the original) but as a completely new file (which utterly destroys the cryptographic validity of the original).
As an aside, whichever software eventually saved your PDF, it did not only forget to use incremental updates but also created a broken PDF objects xref table.