People believe the disturbances in transmissions to be a result of solar flares.
People believe that the disturbances in transmissions to be a result of solar flares.
People believe the disturbances in transmissions to be a result of solar flares.
People believe that the disturbances in transmissions to be a result of solar flares.
The verb "believe" can take a "that" clause or an infinitive, but if a "that" clause is used, it must be followed by a bonafide [noun] + [verb] pair. The construction "that" + [infinitive] is always grammatically incorrect.