Your boiler has detected that the water returning to it is too hot, so it has safely shut itself down to prevent internal damage.

high limit return temperature probe

What causes this fault?

On the Vokera Linea HE, fault 26 occurs when the internal control system experiences a brief communication delay or a software glitch. It is essentially a 'handshake' error within the electronic boards that causes the boiler to pause operations momentarily while it tries to recalibrate. If the system cannot clear this error on its own, it will stop the boiler from firing to protect the electronics.