Let's have MediaInfo on your problem file. Instructions here https://www.vegascreativesoftware.info/us/forum/faq-how-to-post-mediainfo-and-vegas-pro-file-properties--104561/

Alternatively if you have MPC-HC player installed the info provided in its Properties\MediaInfo for the file will suffice.

@Simone-Tagliati

Thanks, thanks, that's it @EricLNZ

General
Complete name                            : C:\Users\Simone\Downloads\2220559125-81649838-cf5b752b-2d16-4098-a262-ed34bfae07b8.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 12.5 GiB
Duration                                 : 4 h 50 min
Overall bit rate mode                    : Constant
Overall bit rate                         : 6 188 kb/s
Frame rate                               : 60.000 FPS
Writing application                      : Lavf60.3.100
Conformance errors                       : 1
 mdat                                    : Yes
  0x09F00000                             : Yes
   General compliance                    : Element size 13471228646 is more than maximal permitted size 13434003622 (offset 0x38)

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.2
Format settings                          : CABAC / 4 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 4 frames
Format settings, GOP                     : M=3, N=120
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 4 h 50 min
Source duration                          : 4 h 50 min
Bit rate mode                            : Constant
Bit rate                                 : 6 003 kb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 60.000 FPS
Minimum frame rate                       : 58.824 FPS
Maximum frame rate                       : 62.500 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.048
Stream size                              : 12.2 GiB (97%)
Source stream size                       : 12.2 GiB (97%)
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709
mdhd_Duration                            : 17415318
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 4 h 50 min
Bit rate mode                            : Constant
Bit rate                                 : 168 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 349 MiB (3%)
Default                                  : Yes
Alternate group                          : 1

@Simone-Tagliati It looks a little suspect. Try a rewrap with Shutterencoder. GorGaram gives the link a few posts above. A rewrap doesn't rerender so there's no quality loss.

I made a ProRes 422 file about 50 gigabytes in size. It loaded up in Vegas normally and you can work with him. You need to look at what's wrong with the source file.

The advice to convert your Variable Frame Rate video to Constant Frame Rate in Shutter Encoder is correct.

@Simone-Tagliati You might as well take a shot at doing a rewrap... it's not likely to do the trick but it's very quick and you might get lucky. A transcode to keep avc, but change vfr to cfr, is more likely to work but will take longer. Also very likely to work is a ProRes transcode. But file sizes may be an impediment. I just tried a 6mbps avc 4:2:0 HD clip transcoded to various ProRes flavors and got these increases in file sizes: Proxy: 11x, LT: 23x, and 422 Std: 34x... all past the 50 gb range starting with a 12.5 gb clip.

The advice to convert your Variable Frame Rate video to Constant Frame Rate in Shutter Encoder is correct.

The framerate variation is minor so I wouldn't expect Vegas to have a problem with it. So let's see first if a simple rewrap works.

I see the same with VP22, but it doesn't crash, VP20 loads file, VP22 doesn't. Probably going to options/preferences/File IO/ and ticking legacy AVC and restarting will make it play.

Also VP22 gets the frame rate wrong, VP20 says 60fps = correct, VP22 says 59.94 = incorrect

I see the same

@UltraVista Please clarify what file you are using to see the same.