Im having an issue trying to display a portal report in SAIL.

...column data array had 28 items.

Pease check local!paginginfo given in the todatasubset.

local!pagingInfo: a!pagingInfo(
startIndex: 1,
batchSize: 20
),

Can you try using Index:
a!gridTextColumn(
label: "Assignee",
data: index(local!datasubset.data, "Assignee")
)

Assignees column contains multiple text values separated by ; .
I think it is giving count of assignees is more than the specified batch size. If you want to know the root cause clearly, you can try to print the datasubset and count of Assignees.
I faced similar issue previously, we can solve that by using support rule of concat the assignees .

a!gridTextColumn(
label: "Assignee",
data: apply(rule!SupportRule,index(local!datasubset.data, "Assignee"))
)

Support rule:

If(count(ri!assignees) > 1, joinarray(ri!assignees,","), ri!assignees)

I hope it will solve that issue.

In portal report, you can enclose the column definition using,
tostring(tp!assignees)

I think the tostring() would be the safest plan, as it would remove the multiple status of the column.

Then if you need individual assignees you can parse the string to get them.

I agree with radhaa, however, it does not solve the problem of applying the rule to individual elements in the list and I have had issues with doing so before. The way I implemented it before, I had a rule with a data parameter of type text and index parameter of type integer so you can apply to the individual elements:

a!gridTextColumn(
label: "Assignee",
data: apply(
rule!SupportRule(data: index(local!datasubset.data, "Assignee", null), index: _),
1 + enumerate(local!datasubset.totalCount)
)

Support rule:

yourRuleToDisplayAssignees(ri!data[ri!index])

See forum.appian.com/.../SAIL_Recipes.html for a SAIL recipe that more fully explains Christine's pattern. They don't directly work with User data, but the concepts translate.

OK.... tostring() will display just the first entry. So in my screenshot previously, it just displayed my name, but not the second user (Even if I removed all the other columns so it is not a display issue).

Trying the solution provided by radhaa, I get the same as above. Just one entry.

Trying the solution provided by christens and harrys...

a!gridTextColumn(
label: "Assignee",
data: if(
local!datasubset.totalCount = 0,
{},
apply(
rule!IIR_displayAssigneeData(
data: index(
local!datasubset.data[4].cells,
"value",
null
),
index: _
),
1 + enumerate(
local!datasubset.totalCount
)
)
)
)


supporting rule:
= tostring(
ri!data[ri!index]
)

If I make the data input of type text, I get the same error as before...

Interface Definition: Expression evaluation error at function a!gridField [line 37]: A grid component [label=“”] has an invalid value for “columns” and “value”. All “data” arrays must not contain more items than the specified “batchSize”, but “batchSize” was 20 and the largest column data array had 23 items.

I've attached the assignees definition if that helps.