User/external breakpoints exist but could not be used

What causes this issue?

When user breakpoints and external breakpoints are created, they are
only active while the logon session in which they were set remains
active. If this logon has been closed, then although the set user
breakpoints and external breakpoints still exist, they cannot be
considered due to technical reasons.
The reason for this is that the debugger screen is always sent to the
terminal with the logon in which the user breakpoints or external
breakpoints were set. If this logon no longer exists, the system assumes
that a debugger screen can no longer be sent.
There is also a limited time frame for how long the user breakpoints and
external breakpoints are active. This is usually set to 2 hours. After
this time has passed, the breakpoints still exist, but are no longer
considered.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

To have the existing user breakpoints and external breakpoints be
considered again, all you have to do is set a new user breakpoint or
external breakpoint in the ABAP Editor or in the ABAP Debugger. All user
breakpoints and external breakpoints that exist for this user are then
added to this logon.

Error message extract from SAP system. Copyright SAP SE.