Not able to import any videos or pictures to Photos from my iphone.

As a test - is it possible to set the "Ignore ownership on this volume" flag as described here: Move your Photos library to save space on your Mac - Apple Support ?

if you cannot set this flag, your external drive is not supported to hold a Photos Library. In addition to the restrictions mentioned by Matti, the volume on the external drive must not have been used for Time Machine backups.

1st question in questions like this: is the external drive formatted as APFS or MacOS Extended as it should (apparently the Photos library is stored there, right)? Non-Mac formats like exFAT or network shares are not supported.

KasiaSwierczynska wrote:

Thank you for trying to help.

My external disk had Photos for a few years and everything was working perfectly with storing and importing pictures to Photos until now. Would the format of my external disk suddenly change by itself?

Just check the format to make sure it is correct because inadvertently using non-Mac formats might corrupt the library. Some macOS versions have been more strict about this and I wish Photos would always have more terse warnings about the incompatibility and possible data loss.

KasiaSwierczynska wrote: …I don't know why everything was working … all those years and now it stopped.

That's what my father said before his first heart attack. But corruption can creep in slowly until it blows.

Mostly, you've been lucky. But as Photos has added more and more features, especially things like face scanning and things like that, the better format has become more and more important.

You should immediately stop using Photos from that drive to avoid damaging the Library.