The input field contains prohibited characters (already deleted)

What causes this issue?

The input field contains prohibited characters. These are generally
characters that your frontend can process, but which are not permitted
in the R/3 database.
There can be several reasons for this:
For any language area, the number of characters allowed by ISO,
Microsoft, IBM, Apple etc. is often similar but rarely exactly the
same. Frontends can often specify and display special characters in
addition to those contained in the ISO character sets.
If you are working on a frontend whose language area is not suitable
for the R/3 System onto which you have logged on, there are several
characters on your keyboard which cannot be used.

System Response

The R/3 System replaces these prohibited characters with other
permitted characters and displays this error message (00.195 or
00.077). The system positions the cursor in the line or field in which
the prohibited character was found.

How to fix this error?

If you agree with the proposal, you should re-choose the last function
key (often [Enter].)
If you do not agree with the proposal, you should edit the data. In
doing so only use permitted characters or the system will display the
error message again.
If you cannot display texts containing characters that are permitted by
your frontend, you should end the current transaction at a suitable
point (usually using the [Esc] key or [F12].)
There may be another frontend that is able to process these characters.

Error message extract from SAP system. Copyright SAP SE.