Hello Ananymous I had to replace mine after several years of operation (fan was making extreme noise) and it still runs reliably through almost 24/7. Now longer than what was installed when I bought it.

No, that fan is 40cm x 40cm, it's a case fan. AFAIK, you cannot replace the fan in the PSU itself. It's not safe to disassemble the PSU, you'll need to by a new PSU if the fan in yours is no longer working

But there are also cables attached. Cables with matching plugs, as long as it is the right power supply unit.

Unfortunately not. This product is compatible with the following models: DS1513+, DS1515+, DS1813+, DS1815+, DS2015xs, RS815+, DS1517, DS1817, RS1219+.

Surely because of soldering. With the 1515+, the cables should be plugged in, similar to a PC. https://www.easy-tecs.de/nas-systeme/synology/ersatzteile/4396/synology-netzteil-psu-250w-3-250w-fuer-ds1513-ds1515-ds1813-ds1815-ds2015xs-rs815 Next, are you sure that the power supply is defective? The 1515+ seems to have a problem with a transistor. https://www.synology-forum.de/threads/ds1515-startet-nicht-mehr-laesst-sich-nicht-einschalten.90668/page-3#post-781595 https://www.synology-forum.de/threads/ds1515-startet-nicht-mehr-laesst-sich-nicht-einschalten.90668/page-8#post-855848

The compatible spare part would be the PSU 250W_5, the original PSU 250W_1 is no longer sold. The PSU 250W_3 is larger than the two mentioned and may not fit in the case.

oui : https://www.synology.com/en-global/products/spare_parts?search_by=category&category=PSU

Doesn't the DS411 have an external power supply? -> Synology Power Supply to DS4xx