I checked my phones files as my phones are set for Most compatible

As I wrote I can add some of these files to a timeline and then suddenly one I add is completely black.

@younso .MOV H.264 is a fairly recent addition to iPhones, I'm not sure how VP 11/13 would handle them.

As you have a range of files with different formats you'll need to work out if it's a particular one that gets the black screen.

@younso your files from iPhone and GoPro are progressive and not interlaced. Also the files from iPhone are variable framerate and do not match the constant framerate of the Gopro. Also resolutions do not match. I cannot remember that Vegas 11 or 13 was able to handle all this differences.

@younso .MOV H.264 is a fairly recent addition to iPhones, I'm not sure how VP 11/13 would handle them.

As you have a range of files with different formats you'll need to work out if it's a particular one that gets the black screen.

that's the problem, I can add say a couple of GoPro files and then suddenly all subsequent one have the problem. All of these files were taken on the same day and GoPro and have been treated identical by my computer.

@younso .MOV H.264 is a fairly recent addition to iPhones, I'm not sure how VP 11/13 would handle them.

As you have a range of files with different formats you'll need to work out if it's a particular one that gets the black screen.

that's the problem, I can add say a couple of GoPro files and then suddenly all subsequent one have the problem. All of these files were taken on the same day and GoPro and have been treated identical by my computer.

Ok, but if I am trying to create projects that either only contain Iphone files or only Go Pro files would that make things easier?

... if I am trying to create projects that either only contain Iphone files or only Go Pro files would that make things easier?

The problem likely lies with the iphone video (or almost any smart phone for that matter) because phone video is almost always recorded in Variable Frame Rate (VFR) so even if you've set the frame rate to, say, 29.95 fps, the actual recorded frame rate can vary from a few frames above and below the set frame rate to wildly different frame rates which could be as few as 10 fps up to 40 fps (+ or -). The best way to find out is to use a detailed MediaInfo report (a free app - https://www.vegascreativesoftware.info/us/forum/faq-how-to-post-mediainfo-and-vegas-pro-file-properties--104561/ - to find out what the upper and lower frame rates are in your VFR phone video.

A minor difference in upper and lower frame rates shouldn't cause a problem in Vegas Pro (with recent versions anyway) but the best way to solve VFR problems is to convert the VFR video to Constant Frame Rate (CFR) in a free transcoder such as the already mentioned Shutter Encoder. When transcoding, it would be best to transcode to PAL 25/50 fps to match your GoPro CFR video - or otherwise transcode your PAL GoPro video to NTSC 29.95 to match your phone video if NTSC frame rates are your preferred fps. While recent Vegas Pro versions can do this with mixed fps rate on the timeline, you'd need to experiment to see how successful this is in your older Vegas Pro versions.

Additionally, I'm not sure why your phone audio recording format is 32 bit float, a format that is not to my knowledge supported in any version of Vegas Pro. 32 float is supposed to be supported in Sound Forge 16 - 18 but I've not been successful in getting my Rode NT1 5th gen 32 float microphone to record in SF16 or SF18 (trial) - but it successfully records in 32 bit float in Audacity.

I think you have more of a chance using 13 than 11. I mixed GoPro and DJI h264 footage in 12 and Canon .mov AVC without issue.

Your hardware isn't that old so if you tried 23 I think you'd have a better editing experience and shouldn't have black screens.

@younso

1. Do you have Apple Quicktime loaded on your system?
2. Is Quicktime used to decode some or all of the media from the iPhone devices?

You said: "...after a while an added video will get a black screen but the audio is still there."

There was an issue with the Quicktime for Windows application, way back when, that led to Apple no longer supporting it and everyone else deprecating it. (It was a 32bit application and had limited ability to handle large address spaces.) You have likely re-discovered the bug at this late point in history.

If this is the source of your "black screen" problem, then converting the media to a different video format would probably circumvent the issue.

Doing the conversion in Vegas Pro 13 likely will not circumvent any issues you might have with variable frame rate video and/or 32bit float audio.

If you upload a sample of each of your source files to a cloud share site (Google Drive, Dropbox, OneDrive, etc.) and post the link

here on this site, others can suggest a rational workaround.

Thanks to everyone for trying to help me. I managed to get hold of Vegas Pro 23 and after some testing last night it seems my problems are solved. I managed to add numerous of GoPro videos to a new project, far exceeding what I had been able to do in 11 or 13 Pro, managed to add transitions and markers as well as keeping them when being exported from Vegas and importing into DVDA Pro 5.2.