Wall cracks: major problem or just plaster?

Once damp plastering dries out it usually cracks and leaves stains. As you have now sorted the damp I would think it will now be the case of skimming the wall and decorating. I would still advise you to get a trade in to check it

This could be due to movement an external cracks running through into the internal space. This would be an external fix with Harley fix bars proxy resin into the Pointing joint. One bar every four courses but a 1 m 500 through either of the crack then it will have to be a repair internally.

Depending on the pattern of the crack(s), size, length this could be of a minor structural failure however the probable cause of this crack appearing in the window region is the quick drying of the plastering on the inside. As you had prior damp issues before rendering outside, probably the wall around the window (and not limited to) was constantly damp and now with the issue solved on the outside, the inside dried quickly and due to shrinkage of the plaster these cracks appeared.

hi it is either subsidence or movement that is the worse case you would need to cut a section of the plaster out to see if it is an hair line crack or not also take a look outside to see if the cracks are showing in the same area outside if it is then heli bars and resin would be needed to be stitched into the brickwork, a surveyor would also advise that. If its just an hairline non structural cracking then the crack could be opened up and either resin the crack or filler it, but you really need a builder in to advise and if your still unsure take advise from a surveyor. I hope this answers your question. I'm always open for advice when it comes to that sort of work as I've done subsidence work for many years.

Cracks on the walls are caused by caused by few factors. However, the ones that you describing is likely caused by plaster itself or minor settlements. Only if they are wider than 3mm and continue to widen then they have to be monitored and checked by a structural engineer.