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I believe I've just hit the same issue

Error converting lexical node to HTML: TypeError: Cannot read properties of undefined (reading 'startsWith')
node: {
 id: '68f5dcd3fa95823a6989a5c6',
 type: 'upload',
 value: {
 id: '9e085dc7-db63-4e32-86ef-74c7bb1df81d',
 alt: 'Sun station',
 url: '/api/media/file/Sun_station.webp',
 filename: 'Sun_station.webp'
 },
 fields: null,
 format: '',
 version: 3,
 relationTo: 'media'
}

... with the following editor state:

{
 "root": {
 "type": "root",
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "tag": "h1",
 "type": "heading",
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "Hello from team!",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 1,
 "version": 1
 }
 ],
 "direction": null,
 "textFormat": 1
 },
 {
 "type": "paragraph",
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "Nice to see you",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 0,
 "version": 1
 }
 ],
 "direction": null,
 "textStyle": "",
 "textFormat": 0
 },
 {
 "type": "paragraph",
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "Here are the reasons:",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 0,
 "version": 1
 }
 ],
 "direction": null,
 "textStyle": "",
 "textFormat": 0
 },
 {
 "tag": "ol",
 "type": "list",
 "start": 1,
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "type": "listitem",
 "value": 1,
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "Blah blah.",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 0,
 "version": 1
 }
 ],
 "direction": null
 },
 {
 "type": "listitem",
 "value": 2,
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "Sun station",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 0,
 "version": 1
 }
 ],
 "direction": null
 }
 ],
 "listType": "number",
 "direction": null
 },
 {
 "id": "68f5dcd3fa95823a6989a5c6",
 "type": "upload",
 "value": "9e085dc7-db63-4e32-86ef-74c7bb1df81d",
 "fields": null,
 "format": "",
 "version": 3,
 "relationTo": "media"
 },
 {
 "type": "quote",
 "format": "",
 "indent": 0,
 "version": 1,
 "children": [
 {
 "mode": "normal",
 "text": "The past is past, now, but that’s... you know, that’s okay! It’s never really gone completely. The future is always built on the past, even if we won’t get to see it.",
 "type": "text",
 "style": "",
 "detail": 0,
 "format": 0,
 "version": 1
 }
 ],
 "direction": null
 }
 ],
 "direction": null
 }
}

... using the officially recommended way of conversion:

 const payload = await getPayload({ config: payloadConfig })
 const html = await convertLexicalToHTMLAsync({
 data,
 populate: await getPayloadPopulateFn({
 payload,
 depth,
 currentDepth: 0,
 }),
 })

I've spent some time reading the codebase and from what I understand, the relationship field stored in Lexical editor state gets populated with partial data when editor state gets selected prior, so the actual state stored in the database...

 {
 "id": "68f5dcd3fa95823a6989a5c6",
 "type": "upload",
 "value": "9e085dc7-db63-4e32-86ef-74c7bb1df81d", // <--- Only ID here.
 "fields": null,
 "format": "",
 "version": 3,
 "relationTo": "media"
 },

... somehow turns into

{
 id: '68f5dcd3fa95823a6989a5c6',
 type: 'upload',
 value: { // <--- Partially populated data.
 id: '9e085dc7-db63-4e32-86ef-74c7bb1df81d',
 alt: null,
 url: '/api/media/file/Sun_station-1.webp',
 filename: 'Sun_station-1.webp'
 },
 fields: null,
 format: '',
 version: 3,
 relationTo: 'media'
}

... so the converter assumes all the necessary data is present and fails to call .mimeType.startsWith() below:

I assume this can be worked around by not populating the relationship field which is suboptimal.

It's quite unexpected to me that Payload somehow inspects the Lexical editor state for relationships and populates them beforehand.

I converted this into a discussion, as this is expected behavior - check out this section of the docs.

By default, convertLexicalToHTML expects fully populated data (e.g. uploads, links, etc.). If you need to dynamically fetch and populate those nodes, use the async variant, convertLexicalToHTMLAsync, from @payloadcms/richtext-lexical/html-async. You must provide a populate function

Lexical upload/relationship/internal link nodes behave the exact same way as payload upload/relationship fields - when you fetch the data with a depth > 0, the related document value will automatically be populated.

The synchronous html converter requires you to pass this populated data, as the populated document contains data required to output the complete HTML (e.g. url). Just the ID is not enough to know how to render an image. It also cannot fetch the image document for you, as it's a synchronous function - hence it has to expect populated data.

If you want the converter to fetch the document for you, you can use the asynchronous version of the HTML converter and pass a populate function.