Just to share with everyone that removing all activities from the faulty workflow - as @Sami_Rajput reported - and then creating a new one from scratch and then copying the activities back one by one did the trick for me.
Curious to understand what caused the problem and if it is probe to occur again in other legacy automation projects of mine.
Anil_G
ashokkarale
jinal.shah
Gautham_Pattabiraman
postwick
chandreshsinh.jadeja
vrdabberu
Ajay_Mishra
sven.wullum1
Vyshnavi_Nalumachu